Suggest including <stdbool.h> for bool, true and false
Currently gcc suggests to use _Bool instead of bool and doesn't give
any suggestions when true or false are used, but undefined. This patch
makes it so that (for C99 or higher) a fixit hint is emitted to include
<stdbool.h>.
gcc/c-family/ChangeLog:
* known-headers.cc (get_stdlib_header_for_name): Return
"<stdbool.h>" for "bool", "true" or "false" when STDLIB_C and
flag_isoc99.