[llvm] [DenseMap] Fix build failure in lookup_or (PR #142268)

[Aaron Puchert via llvm-commits]

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@@ -226,6 +226,12 @@ class DenseMapBase : public DebugEpochBase {
     return Default;
   }
 
+  ValueT lookup_or(const_arg_type_t<KeyT> Val, const ValueT &Default) const {
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aaronpuchert wrote:

With `U` being a template parameter, this is no longer an rvalue reference but a [universal/forwarding reference](https://www.cppreference.com/w/cpp/language/reference.html#Forwarding_references). Such a reference can bind to lvalues, in which case `U` will be derived as lvalue reference type using [reference collapsing](https://www.cppreference.com/w/cpp/language/reference.html#Reference_collapsing). So this should work.

To be honest, I have no idea what the default argument does, since it seems like this should always be deduced.

https://github.com/llvm/llvm-project/pull/142268