Thread a source location through canImport evaluation in CompilerBuildConfiguration

Unlike all of the other build configuration checks, `canImport` has
side effects including the emission of various diagnostics. Thread a
source location through here so the diagnostics end up on the right
line.

Author: DougGregor

include/swift/AST/ASTBridging.h

@@ -252,9 +252,10 @@ enum ENUM_EXTENSIBILITY_ATTR(closed) BridgedCanImportVersion : size_t {
   CanImportUnderlyingVersion,
 };
 
-SWIFT_NAME("BridgedASTContext.canImport(self:importPath:versionKind:versionComponents:numVersionComponents:)")
+SWIFT_NAME("BridgedASTContext.canImport(self:importPath:location:versionKind:versionComponents:numVersionComponents:)")
 bool BridgedASTContext_canImport(BridgedASTContext cContext,
                                  BridgedStringRef importPath,
+                                 BridgedSourceLoc canImportLoc,
                                  BridgedCanImportVersion versionKind,
                                  const SwiftInt * _Nullable versionComponents,
                                  SwiftInt numVersionComponents);

lib/AST/ASTBridging.cpp

@@ -216,6 +216,7 @@ SwiftInt BridgedASTContext_langOptsGetTargetAtomicBitWidths(BridgedASTContext cC
 
 bool BridgedASTContext_canImport(BridgedASTContext cContext,
                                  BridgedStringRef importPath,
+                                 BridgedSourceLoc canImportLoc,
                                  BridgedCanImportVersion versionKind,
                                  const SwiftInt * _Nullable versionComponents,
                                  SwiftInt numVersionComponents) {
@@ -240,15 +241,11 @@ bool BridgedASTContext_canImport(BridgedASTContext cContext,
     break;
   }
 
-  // FIXME: The source location here is empty because build configurations
-  // are supposed to be completely separated from source code. We could re-plumb
-  // things to have any errors reported up through the "canImportModule"
-  // API.
   ImportPath::Module::Builder builder(
       cContext.unbridged(), importPath.unbridged(), /*separator=*/'.',
-      SourceLoc());
+      canImportLoc.unbridged());
   return cContext.unbridged().canImportModule(
-      builder.get(), SourceLoc(), version,
+      builder.get(), canImportLoc.unbridged(), version,
       versionKind == CanImportUnderlyingVersion);
 }
 

lib/AST/ASTContext.cpp

@@ -2598,7 +2598,10 @@ bool ASTContext::canImportModuleImpl(ImportPath::Module ModuleName,
     // The module version could not be parsed from the preferred source for
     // this query. Diagnose and treat the query as if it succeeded.
     auto mID = ModuleName[0];
-    Diags.diagnose(mID.Loc, diag::cannot_find_project_version,
+    auto diagLoc = mID.Loc;
+    if (mID.Loc.isInvalid())
+      diagLoc = loc;
+    Diags.diagnose(diagLoc, diag::cannot_find_project_version,
                    getModuleVersionKindString(bestVersionInfo), mID.Item.str());
     return true;
   }

lib/ASTGen/Sources/ASTGen/CompilerBuildConfiguration.swift

@@ -11,13 +11,16 @@
 //===----------------------------------------------------------------------===//
 
 import ASTBridging
+import SwiftDiagnostics
 import SwiftIfConfig
+import SwiftParser
 import SwiftSyntax
 
 /// A build configuration that uses the compiler's ASTContext to answer
 /// queries.
 struct CompilerBuildConfiguration: BuildConfiguration {
   let ctx: BridgedASTContext
+  let conditionLoc: BridgedSourceLoc
 
   func isCustomConditionSet(name: String) throws -> Bool {
     var name = name
@@ -63,6 +66,7 @@ struct CompilerBuildConfiguration: BuildConfiguration {
       versionComponents.withUnsafeBufferPointer { versionComponentsBuf in
         ctx.canImport(
           importPath: bridgedImportPathStr,
+          location: conditionLoc,
           versionKind: cVersionKind,
           versionComponents: versionComponentsBuf.baseAddress,
           numVersionComponents: versionComponentsBuf.count
@@ -154,12 +158,15 @@ public func configuredRegions(
   cRegionsOut: UnsafeMutablePointer<UnsafeMutablePointer<BridgedIfConfigClauseRangeInfo>?>
 ) -> Int {
   let sourceFilePtr = sourceFilePtr.bindMemory(to: ExportedSourceFile.self, capacity: 1)
-  let configuration = CompilerBuildConfiguration(ctx: astContext)
+  let configuration = CompilerBuildConfiguration(
+    ctx: astContext,
+    conditionLoc: sourceFilePtr.pointee.sourceLoc(at: AbsolutePosition(utf8Offset: 0))
+  )
   let regions = sourceFilePtr.pointee.syntax.configuredRegions(in: configuration)
 
   var cRegions: [BridgedIfConfigClauseRangeInfo] = []
 
-  // Keep track of the enclosing #ifs so that we can emit and "#endif" directive
+  // Keep track of the enclosing #ifs so that we can emit an "#endif" directive
   // right before moving on to the next #if (and at the end).
   var ifConfigStack: [IfConfigDeclSyntax] = []