[Targets] Define __BOOL_DEFINED for Windows targets in C++ mode

MSVC 4.2 didn't have bool as a builtin type but MSVC 5.0 does.  When
they added it, they added a macro (__BOOL_DEFINED) which allows build
scripts and the like to know if they should provide their own bool.

Clang always supports bool as a builtin type in C++ mode.

llvm-svn: 242307