Parse vector bool when stdbool.h and altivec.h are included

Currently when including stdbool.h and altivec.h declaration of `vector bool` leads to
errors due to `bool` being expanded to '_Bool`. This patch allows the parser
to recognize `_Bool`.

Reviewed By: hubert.reinterpretcast, Everybody0523

Differential Revision: https://reviews.llvm.org/D102064