[libc] [llvm] [libc] Add user defined literals to ease testing (PR #81267)

[Guillaume Chatelet via llvm-commits]

gchatelet wrote:

@lntue @michaelrj-google I think this could be useful for testing purposes as we need to deal with the impedance mismatch between C++ types, extensions (i.e., `__uint128_t`) and `BigInt` types. Do you think it bears its weight?

Ideally the functions should be `consteval` but we're not C++20 yet.
For now I think it should be used in tests only. I'm not sure how much it'd increase compile time when we have a lot of them.

https://github.com/llvm/llvm-project/pull/81267