[Serialization] Intro ModuleFile::getSourceLoc()

Generate a fake empty buffer to return a SourceLoc pointing to the
beginning of a swiftmodule file.

Author: xymus

lib/Serialization/Deserialization.cpp

@@ -179,6 +179,15 @@ void ModuleFile::fatal(llvm::Error error) const {
   Core->fatal(diagnoseFatal(std::move(error)));
 }
 
+SourceLoc ModuleFile::getSourceLoc() const {
+  auto &SourceMgr = getContext().Diags.SourceMgr;
+  auto filename = getModuleFilename();
+  auto bufferID = SourceMgr.getIDForBufferIdentifier(filename);
+  if (!bufferID)
+    bufferID = SourceMgr.addMemBufferCopy(StringRef(), filename);
+  return SourceMgr.getLocForBufferStart(*bufferID);
+}
+
 llvm::Error ModuleFile::diagnoseFatal(llvm::Error error) const {
   if (FileContext)
     getContext().Diags.diagnose(SourceLoc(), diag::serialization_fatal,

lib/Serialization/ModuleFile.h

@@ -667,6 +667,9 @@ class ModuleFile
   getTransitiveLoadingBehavior(const Dependency &dependency,
                                bool forTestable) const;
 
+  /// Generate a \c SourceLoc pointing at the loaded swiftmodule file.
+  SourceLoc getSourceLoc() const;
+
   /// Returns `true` if there is a buffer that might contain source code where
   /// other parts of the compiler could have emitted diagnostics, to indicate
   /// that the object must be kept alive as long as the diagnostics exist.