Ensure Ident_GNU_final is properly initialized in the Parser Initialize function

The recent change implementing __final forgot to initialize a variable.
This was caught by the Memory Sanitizer.

Properly initialize the value to nullptr to ensure proper memory reads.

Patch by Erich Keane!

Differential Revision: https://reviews.llvm.org/D22970

llvm-svn: 277206

clang/lib/Parse/Parser.cpp

@@ -473,6 +473,7 @@ void Parser::Initialize() {
   Ident_final = nullptr;
   Ident_sealed = nullptr;
   Ident_override = nullptr;
+  Ident_GNU_final = nullptr;
 
   Ident_super = &PP.getIdentifierTable().get("super");