Make ASTUnit structure stable with NDEBUG

ASTUnit instances are allocated infrequently so it's fine to keep this field
around in all build configurations.

Assigns null to silence -Wunused-private-field in Release.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195419 91177308-0d34-0410-b5e6-96231b3b80d8