Wrap the #include of <stdbool.h> in an #ifndef __cplusplus.

This should fix the MSVC build.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191357 91177308-0d34-0410-b5e6-96231b3b80d8
Try again to fix the MSVC build.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191359 91177308-0d34-0410-b5e6-96231b3b80d8