Provide the buffer ID for source files created for C swift_attr attributes

When rendering a swift_attr attribute for parsing, we were creating both
the backing buffer and a source file, but not providing the buffer's ID
to the source file, so we couldn't find the source code again. Fix that.

While here, also register the source file with the module, so we can
find the source file again based on location.

Author: DougGregor

lib/ClangImporter/ImportDecl.cpp

@@ -8063,14 +8063,18 @@ unsigned ClangImporter::Implementation::getClangSwiftAttrSourceBuffer(
 }
 
 SourceFile &ClangImporter::Implementation::getClangSwiftAttrSourceFile(
-    ModuleDecl &module) {
+    ModuleDecl &module, unsigned bufferID) {
   auto known = ClangSwiftAttrSourceFiles.find(&module);
   if (known != ClangSwiftAttrSourceFiles.end())
     return *known->second;
 
   auto sourceFile = new (SwiftContext)
-      SourceFile(module, SourceFileKind::Library, std::nullopt);
+      SourceFile(module, SourceFileKind::Library, bufferID);
   ClangSwiftAttrSourceFiles.insert({&module, sourceFile});
+
+  // Record this attribute in the module.
+  module.addAuxiliaryFile(*sourceFile);
+
   return *sourceFile;
 }
 
@@ -8228,7 +8232,7 @@ ClangImporter::Implementation::importSwiftAttrAttributes(Decl *MappedDecl) {
 
       // Dig out a source file we can use for parsing.
       auto &sourceFile = getClangSwiftAttrSourceFile(
-          *MappedDecl->getDeclContext()->getParentModule());
+          *MappedDecl->getDeclContext()->getParentModule(), bufferID);
 
       // Spin up a parser.
       swift::Parser parser(

lib/ClangImporter/ImporterImpl.h

@@ -1062,7 +1062,7 @@ class LLVM_LIBRARY_VISIBILITY ClangImporter::Implementation
 
   /// Retrieve the placeholder source file for use in parsing Swift attributes
   /// in the given module.
-  SourceFile &getClangSwiftAttrSourceFile(ModuleDecl &module);
+  SourceFile &getClangSwiftAttrSourceFile(ModuleDecl &module, unsigned bufferID);
 
   /// Utility function to import Clang attributes from a source Swift decl to
   /// synthesized Swift decl.