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 2b8b90a, but renaming the
macro in line with LLVM's other include guards seems like a better fix.

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

llvm/lib/Support/regex_impl.h

@@ -35,8 +35,8 @@
  *	@(#)regex.h	8.1 (Berkeley) 6/2/93
  */
 
-#ifndef _REGEX_H_
-#define	_REGEX_H_
+#ifndef LLVM_SUPPORT_REGEX_IMPL_H
+#define LLVM_SUPPORT_REGEX_IMPL_H
 
 #include <sys/types.h>
 typedef off_t llvm_regoff_t;
@@ -105,4 +105,4 @@ size_t  llvm_strlcpy(char *dst, const char *src, size_t siz);
 }
 #endif
 
-#endif /* !_REGEX_H_ */
+#endif /* LLVM_SUPPORT_REGEX_IMPL_H */