Fix a -Wmicrosoft warning about an unrepresentable enum value

In MSVC, enums are always signed unless you explicitly specify the type.

llvm-svn: 200719
diff --git a/clang/lib/Sema/SemaAttr.cpp b/clang/lib/Sema/SemaAttr.cpp
index 0214ba4..499f958 100644
--- a/clang/lib/Sema/SemaAttr.cpp
+++ b/clang/lib/Sema/SemaAttr.cpp
@@ -327,7 +327,7 @@
 }
 
 typedef std::vector<std::pair<unsigned, SourceLocation> > VisStack;
-enum { NoVisibility = (unsigned) -1 };
+enum LLVM_ENUM_INT_TYPE(unsigned) { NoVisibility = ~0U };
 
 void Sema::AddPushedVisibilityAttribute(Decl *D) {
   if (!VisContext)