[AST] Never call 'getSerializedLocs()' on non-serialized file unit

For example 'SourceFile' can handle all operations inside
'getSerializedLocs()', but it unnecessarily loads source content in a
new buffer, and resolve 'SourceLoc' from the line and the column.
And the result is completely useless.

rdar://problem/75010520

Author: rintaro

lib/AST/Decl.cpp

@@ -600,6 +600,9 @@ const Decl::CachedExternalSourceLocs *Decl::getSerializedLocs() const {
     return CachedSerializedLocs;
   }
   auto *File = cast<FileUnit>(getDeclContext()->getModuleScopeContext());
+  assert(File->getKind() == FileUnitKind::SerializedAST &&
+         "getSerializedLocs() should only be called on decls in "
+         "a 'SerializedASTFile'");
   auto Locs = File->getBasicLocsForDecl(this);
   if (!Locs.hasValue()) {
     static const Decl::CachedExternalSourceLocs NullLocs{};

lib/AST/RawComment.cpp

@@ -147,9 +147,14 @@ RawComment Decl::getRawComment(bool SerializedOK) const {
     return Result;
   }
 
-  // Ask the parent module.
-  if (auto *Unit =
-          dyn_cast<FileUnit>(this->getDeclContext()->getModuleScopeContext())) {
+  if (!getDeclContext())
+    return RawComment();
+  auto *Unit = dyn_cast<FileUnit>(getDeclContext()->getModuleScopeContext());
+  if (!Unit)
+    return RawComment();
+
+  switch (Unit->getKind()) {
+  case FileUnitKind::SerializedAST: {
     if (SerializedOK) {
       if (const auto *CachedLocs = getSerializedLocs()) {
         if (!CachedLocs->DocRanges.empty()) {
@@ -158,8 +163,8 @@ RawComment Decl::getRawComment(bool SerializedOK) const {
             if (Range.isValid()) {
               SRCs.push_back({ Range, Context.SourceMgr });
             } else {
-              // if we've run into an invalid range, don't bother trying to load any of
-              // the other comments
+              // if we've run into an invalid range, don't bother trying to load
+              // any of the other comments
               SRCs.clear();
               break;
             }
@@ -178,10 +183,17 @@ RawComment Decl::getRawComment(bool SerializedOK) const {
       Context.setRawComment(this, C->Raw);
       return C->Raw;
     }
-  }
 
-  // Give up.
-  return RawComment();
+    return RawComment();
+  }
+  case FileUnitKind::Source:
+  case FileUnitKind::Builtin:
+  case FileUnitKind::Synthesized:
+  case FileUnitKind::ClangModule:
+  case FileUnitKind::DWARFModule:
+    return RawComment();
+  }
+  llvm_unreachable("invalid file kind");
 }
 
 static const Decl* getGroupDecl(const Decl *D) {