[Clang] Instantiate local constexpr functions eagerly (#95660)
We had a code path in `Sema::MarkFunctionReferenced()` that deferred
local lambda instantiation even for constexprs. This resulted in any
calls to them referring to function decls that lack bodies and hence
failures at constant evaluation.
The issue doesn't occur when the lambda has no explicit return type
because the return type deduction requires instantiation.
Fixes https://github.com/llvm/llvm-project/issues/35052
Fixes https://github.com/llvm/llvm-project/issues/94849
