[Clang][C++2b] P2242R3: Non-literal variables [...] in constexpr

Allow goto, labelled statements as well as `static`, `thread_local`, and
non-literal variables in `constexpr` functions.

As specified. for all of the above (except labelled statements) constant
evaluation of the construct still fails.

For `constexpr` bodies, the proposal is implemented with diagnostics as
a language extension in older language modes. For determination of
whether a lambda body satisfies the requirements for a constexpr
function, the proposal is implemented only in C++2b mode to retain the
semantics of older modes for programs conforming to them.

Reviewed By: aaron.ballman, hubert.reinterpretcast, erichkeane

Differential Revision: https://reviews.llvm.org/D111400