[SimplifyLibCalls] Add initial support for non-8-bit bytes
The patch makes CharWidth argument of `getStringLength` mandatory
and ensures the correct values are passed in most cases.
This is *not* a complete support for unusual byte widths in
SimplifyLibCalls since `getConstantStringInfo` returns false for those.
The code guarded by `getConstantStringInfo` returning true is unchanged
because the changes are currently not testable.
