In stdbool.h, define bool, false, true only in gnu++98

GCC has meanwhile corrected that with the similar
<https://gcc.gnu.org/viewcvs/gcc?view=revision&revision=216679> "C++11
explicitly forbids macros for bool, true and false."

Differential Revision: https://reviews.llvm.org/D40167

llvm-svn: 320135