Add static cast to unsigned char whenever a character classification function is called with a signed char argument, in order to avoid assertions in Windows Debug configuration.

llvm-svn: 175006
diff --git a/llvm/lib/Support/Windows/Path.inc b/llvm/lib/Support/Windows/Path.inc
index 98d8a18..f4898e6 100644
--- a/llvm/lib/Support/Windows/Path.inc
+++ b/llvm/lib/Support/Windows/Path.inc
@@ -82,7 +82,7 @@
   pos = path.rfind(':',len);
   size_t rootslash = 0;
   if (pos != std::string::npos) {
-    if (pos != 1 || !isalpha(path[0]) || len < 3)
+    if (pos != 1 || !isalpha(static_cast<unsigned char>(path[0])) || len < 3)
       return false;
       rootslash = 2;
   }