lib/Support/regcomp.c: Fix cygwin warning.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@127241 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Support/regcomp.c b/lib/Support/regcomp.c
index cd018d5..46c91a9 100644
--- a/lib/Support/regcomp.c
+++ b/lib/Support/regcomp.c
@@ -780,7 +780,7 @@
 	const char *u;
 	char c;
 
-	while (MORE() && isalpha(PEEK()))
+	while (MORE() && isalpha((uch)PEEK()))
 		NEXT();
 	len = p->next - sp;
 	for (cp = cclasses; cp->name != NULL; cp++)