Explicitly request unsigned enum types when desired

This fixes repeated -Wmicrosoft warnings when self-hosting clang on
Windows, and gets us real unsigned enum types with MSVC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192227 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/RegAllocFast.cpp

@@ -144,7 +144,7 @@ namespace {
     // not be erased.
     bool isBulkSpilling;
 
-    enum {
+    enum LLVM_ENUM_INT_TYPE(unsigned) {
       spillClean = 1,
       spillDirty = 100,
       spillImpossible = ~0u