clang: Introduce -fexperimental-max-bitint-width

This splits of the introduction of -fexperimental-max-bitint-width
from https://reviews.llvm.org/D122234
because that PR is still blocked on discussions on the backend side.

I was asked [0] to upstream at least the flag.

[0] https://github.com/llvm/llvm-project/commit/09854f2af3b914b616f29cb640bede3a27cf7c4e#commitcomment-75116619

Differential Revision: https://reviews.llvm.org/D127287