[libc] [llvm] [libc][NFC] Rename `UInt.h` to `big_int.h` and `UInt128.h` to `uint128.h` for consistency (PR #87751)

[Guillaume Chatelet via llvm-commits]

gchatelet wrote:

I don't know what happened during rebase but the PR is all messed up. 
Converting to draft until it's fixed.

https://github.com/llvm/llvm-project/pull/87751