Eliminate DelayedParsingCallbacks.

DelayedParsingCallbacks only had one implementation, for code
completion, which is only used to determine which bodies to skip and
which to delay. Inline that logic into the parser's delay logic and
remove DelayedParsingCallbacks entirely.

lib/Basic/SourceLoc.cpp

@@ -35,6 +35,9 @@ void SourceManager::verifyAllBuffers() const {
 }
 
 SourceLoc SourceManager::getCodeCompletionLoc() const {
+  if (CodeCompletionBufferID == 0U)
+    return SourceLoc();
+
   return getLocForBufferStart(CodeCompletionBufferID)
       .getAdvancedLoc(CodeCompletionOffset);
 }