[DenseMap] Fix MSVC buildbot failure in lookup_or (#142268)

4bf67cd ([DenseMap] Fix constness issues with lookup_or) introduced a
buildbot failure:

  https://lab.llvm.org/buildbot/#/builders/63/builds/6559

The patch deviates from the spec and MSVC complains, where it doesn't
bind an lvalue to an rvalue reference. Fix it by qualifying the argument
of lookup_or with remove_cv_t.

Proof: https://godbolt.org/z/sjTvGMbce

Author: artagnon

llvm/include/llvm/ADT/DenseMap.h

@@ -220,7 +220,8 @@ class DenseMapBase : public DebugEpochBase {
   // Return the entry with the specified key, or \p Default. This variant is
   // useful, because `lookup` cannot be used with non-default-constructible
   // values.
-  ValueT lookup_or(const_arg_type_t<KeyT> Val, ValueT &&Default) const {
+  template <typename U = std::remove_cv_t<ValueT>>
+  ValueT lookup_or(const_arg_type_t<KeyT> Val, U &&Default) const {
     if (const BucketT *Bucket = doFind(Val))
       return Bucket->getSecond();
     return Default;