[DenseMap] Work around a MSVC bug in lookup_or

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

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

This seems to be a bug in MSVC, where it doesn't bind an lvalue to an
rvalue reference. Work around it by introducing a const-lvalue-reference
variant of lookup_or.

Author: artagnon

llvm/include/llvm/ADT/DenseMap.h

@@ -226,6 +226,12 @@ class DenseMapBase : public DebugEpochBase {
     return Default;
   }
 
+  ValueT lookup_or(const_arg_type_t<KeyT> Val, const ValueT &Default) const {
+    if (const BucketT *Bucket = doFind(Val))
+      return Bucket->getSecond();
+    return Default;
+  }
+
   /// at - Return the entry for the specified key, or abort if no such
   /// entry exists.
   const ValueT &at(const_arg_type_t<KeyT> Val) const {