Fix compilation on Darwin with expensive checks.

After r327219 was landed, the bot with expensive checks on GreenDragon
started failing. The problem was missing symbols `regex_t` and
`regmatch_t` in `xlocale/_regex.h`. The latter was included because
after the change in r327219, `random` is needed, which transitively
includes `xlocale.h.` which in turn conditionally includes
`xlocale/_regex.h` when _REGEX_H_ is defined. Because this is the header
guard in `regex_impl.h` and because `regex_impl.h` was included before
the other LLVM includes, `xlocale/_regex.h` was included without the
necessary types being available.

This commit fixes this by moving the include of `regex_impl.h` all the
way down. I also added a comment to stress the significance of its
position.

llvm-svn: 327256

llvm/lib/Support/Regex.cpp

@@ -12,11 +12,16 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Support/Regex.h"
-#include "regex_impl.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/Twine.h"
 #include <string>
+
+// Important this comes last because it defines "_REGEX_H_". At least on
+// Darwin, if included before any header that (transitively) includes
+// xlocale.h, this will cause trouble, because of missing regex-related types.
+#include "regex_impl.h"
+
 using namespace llvm;
 
 Regex::Regex() : preg(nullptr), error(REG_BADPAT) {}