[libcxx] Fix build breakage on mips

Fixes https://bugs.llvm.org/show_bug.cgi?id=43011 caused by
https://reviews.llvm.org/D63284.

Committing as obvious.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@369364 91177308-0d34-0410-b5e6-96231b3b80d8

include/__locale

@@ -409,7 +409,7 @@ public:
     static const mask xdigit = _ISxdigit;
     static const mask blank  = _ISblank;
 #if defined(__mips__)
-    static const mask __regex_word = static_cast<char_class_type>(_ISbit(15));
+    static const mask __regex_word = static_cast<mask>(_ISbit(15));
 #else
     static const mask __regex_word = 0x80;
 #endif