[libc] [llvm] [libc][NFC] Use user defined literals to build 128 and 256 bit constants. (PR #81746)

[Guillaume Chatelet via llvm-commits]

gchatelet wrote:

Most of the change is done via `sed` so it should be safe. I'm running it on an aarch64 machine to make sure all the tests are exercised.

https://github.com/llvm/llvm-project/pull/81746