[libc] fix -Wshift-count-overflow in UInt.h (#74649)

Not that I'm very good at SFINAE, but it seems that conversion operators
are
perhaps difficult to compose with SFINAE. I saw an example that used one
layer
of indirection to have an explicit return type that could then be used
with
enable_if_t.

Link: https://stackoverflow.com/a/7604580
Fixes: #74623