Use LLVM-style include guard in regex_impl.h

The previous include guard (_REGEX_H_) is also used in a macOS SDK
header (xlocale.h), causing potential for trouble. This was previously
addressed in 2b8b90a7, but renaming the
macro in line with LLVM's other include guards seems like a better fix.

Differential revision: https://reviews.llvm.org/D150117