Register the __builtin_va_list_type node when we parse it, rather than

Register the __builtin_va_list_type node when we parse it, rather than
waiting until we think we need it: we didn't catch all of the places
where we actually needed it, and we probably wouldn't ever. Fixes a
C++ PCH crasher.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@115621 91177308-0d34-0410-b5e6-96231b3b80d8