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

[via llvm-commits]

michaelrj-google wrote:

this seems like a useful tool given the difficulties initializing uint128. Currently in the `str_to_long_double_test.cpp` float128 tests, the uint128 values are initialized from two uint64s and some shifts.

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