[Clang importer] Use ParseSourceFileRequest for parsing swift_attr attributes

The Clang importer was directly calling into the parser to parse the
attribute (or modifier) within swift_attr. Aside from being gross, this
isn't possible with ASTGen.

Instead, teach ParseSourceFileRequest to deal with modifiers in the
same way that the Clang importer was hardcoding, and have the Clang
importer pull the attributes/modifiers off of the "missing"
declaration introduced by the request.

One benefit of this approach is that we're only parsing each
swift_attr source buffer once, then cloning the attributes each time
it's used, so we should be doing less work overall.

Fixes rdar://139119159.

Author: DougGregor

include/swift/Parse/Parser.h

@@ -1322,7 +1322,11 @@ class Parser {
   ///
   /// Parsing a floating attribute list will produce a `MissingDecl` with
   /// the attribute list attached.
-  void parseExpandedAttributeList(SmallVectorImpl<ASTNode> &items);
+  ///
+  /// If isFromClangAttribute, we also parse modifiers and suppress any
+  /// diagnostics about bad modifiers.
+  void parseExpandedAttributeList(SmallVectorImpl<ASTNode> &items,
+                                  bool isFromClangAttribute);
 
   /// Parse the result of member macro expansion, which is a floating
   /// member list.

lib/ClangImporter/ImportDecl.cpp

@@ -52,7 +52,6 @@
 #include "swift/ClangImporter/ClangImporterRequests.h"
 #include "swift/ClangImporter/ClangModule.h"
 #include "swift/Parse/Lexer.h"
-#include "swift/Parse/Parser.h"
 #include "swift/Strings.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/Attr.h"
@@ -8281,7 +8280,6 @@ ClangImporter::Implementation::importSwiftAttrAttributes(Decl *MappedDecl) {
 
   std::optional<const clang::SwiftAttrAttr *> seenMainActorAttr;
   const clang::SwiftAttrAttr *seenMutabilityAttr = nullptr;
-  PatternBindingInitializer *initContext = nullptr;
 
   auto importAttrsFromDecl = [&](const clang::NamedDecl *ClangDecl) {
     //
@@ -8406,37 +8404,12 @@ ClangImporter::Implementation::importSwiftAttrAttributes(Decl *MappedDecl) {
           *MappedDecl->getDeclContext()->getParentModule(),
           swiftAttr->getAttribute());
 
-      // Spin up a parser.
-      swift::Parser parser(
-          sourceFile.getBufferID(), sourceFile, &SwiftContext.Diags,
-          nullptr, nullptr);
-      // Prime the lexer.
-      parser.consumeTokenWithoutFeedingReceiver();
-
-      bool hadError = false;
-      if (parser.Tok.is(tok::at_sign)) {
-        SourceLoc atEndLoc = parser.Tok.getRange().getEnd();
-        SourceLoc atLoc = parser.consumeToken(tok::at_sign);
-        hadError = parser
-                       .parseDeclAttribute(MappedDecl->getAttrs(), atLoc,
-                                           atEndLoc, initContext,
-                                           /*isFromClangAttribute=*/true)
-                       .isError();
-      } else {
-        SourceLoc staticLoc;
-        StaticSpellingKind staticSpelling;
-        hadError = parser
-                       .parseDeclModifierList(MappedDecl->getAttrs(), staticLoc,
-                                              staticSpelling,
-                                              /*isFromClangAttribute=*/true)
-                       .isError();
-      }
-
-      if (hadError) {
-        // Complain about the unhandled attribute or modifier.
-        HeaderLoc attrLoc(swiftAttr->getLocation());
-        diagnose(attrLoc, diag::clang_swift_attr_unhandled,
-                 swiftAttr->getAttribute());
+      // Collect the attributes from the synthesized top-level declaration in
+      // the source file.
+      auto topLevelDecls = sourceFile.getTopLevelDecls();
+      for (auto decl : topLevelDecls) {
+        for (auto attr : decl->getAttrs())
+          MappedDecl->getAttrs().add(attr->clone(SwiftContext));
       }
     }
 

lib/Parse/ParseDecl.cpp

@@ -8166,17 +8166,28 @@ void Parser::parseTopLevelAccessors(
     items.push_back(accessor);
 }
 
-void Parser::parseExpandedAttributeList(SmallVectorImpl<ASTNode> &items) {
+void Parser::parseExpandedAttributeList(SmallVectorImpl<ASTNode> &items,
+                                        bool isFromClangAttribute) {
   if (Tok.is(tok::NUM_TOKENS))
     consumeTokenWithoutFeedingReceiver();
 
   DeclAttributes attributes;
   parseDeclAttributeList(attributes);
 
+  // If this is coming from a Clang attribute, also parse modifiers.
+  if (isFromClangAttribute && !Tok.is(tok::eof)) {
+    SourceLoc staticLoc;
+    StaticSpellingKind staticSpelling;
+    parseDeclModifierList(
+        attributes, staticLoc, staticSpelling, isFromClangAttribute);
+  }
+
   // Consume remaining tokens.
   while (!Tok.is(tok::eof)) {
-    diagnose(Tok.getLoc(), diag::unexpected_attribute_expansion,
-             Tok.getText());
+    if (!isFromClangAttribute) {
+      diagnose(Tok.getLoc(), diag::unexpected_attribute_expansion,
+               Tok.getText());
+    }
     consumeToken();
   }
 

lib/Parse/ParseRequests.cpp

@@ -379,10 +379,12 @@ SourceFileParsingResult parseSourceFile(SourceFile &SF) {
     }
 
     case GeneratedSourceInfo::MemberAttributeMacroExpansion:
-    case GeneratedSourceInfo::Attribute: {
-      parser.parseExpandedAttributeList(items);
+      parser.parseExpandedAttributeList(items, /*isFromClangAttribute=*/false);
+      break;
+
+    case GeneratedSourceInfo::Attribute:
+      parser.parseExpandedAttributeList(items, /*isFromClangAttribute=*/true);
       break;
-    }
 
     case GeneratedSourceInfo::PeerMacroExpansion: {
       if (parser.CurDeclContext->isTypeContext()) {