Abstract the details from regex.h a bit more by not allowing people to specify...

Abstract the details from regex.h a bit more by not allowing people to specify compile and execute flags for regular expressions. Also enable better regular expressions if they are available by check if the REG_ENHANCED is available and using it if it is.

Since REG_ENHANCED is available on MacOSX, this allow the use of \d (digits) \b (word boundaries) and much more without affecting other systems.

<rdar://problem/12082562>

llvm-svn: 226704