<xmemory>: Tolerate bogus const-overloading in iterators passed to uninitialized_meow()

[StephanTLavavej]

• Followup to Implement LWG-3918 std::uninitialized_move/_n and guaranteed copy elision #5135.
• Reported [llvm] Input iterators should not const-overload operator* with varying return types llvm/llvm-project#119084.
• Fixes VSO-2318081 / AB#2318081 "[RWC][prod/fe][Regression] 4 projects failed with error C2440: 'initializing': cannot convert from 'const llvm::Value *' to '_Ty'".

In an ideal world with infinite capacity, we would push through RWC fixes as we did with #5105. However:

• 🎅
• These fixes will be more intrusive than adding #include <chrono>.
• I suspect that a fair amount of code will be affected and LLVM is just the first canary, even though uninitialized_meow() is relatively obscure.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.