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] 09854f2af3 (commitcomment-75116619)
Differential Revision: https://reviews.llvm.org/D127287