[clang] Do not attempt to zero-extend _BitInt(1) when not required

`ConvertTypeForMem` doesn't return wider type for _BitInt unless it is
used in a bitfield, so no need to extend when trying to initialize a
global variable.

Fixes https://github.com/llvm/llvm-project/issues/62207

Reviewed By: erichkeane, shafik

Differential Revision: https://reviews.llvm.org/D149436