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 2b8b90a7686858b1d22cae6fcfbd0904135112aa, but renaming the
macro in line with LLVM's other include guards seems like a better fix.
Differential revision: https://reviews.llvm.org/D150117