Ensure Ident_GNU_final is properly initialized in the Parser Initialize function

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@277206 91177308-0d34-0410-b5e6-96231b3b80d8