[SLC] Simplify strcpy and friends with non-zero address spaces

The current logic in TargetLibraryInfoImpl::getLibFunc() was only treating
strcpy, etc. with i8* arguments in address space zero as a valid library
function. However, in the CHERI and Morello targets we expect all libc
functions to use address space 200 arguments.

This commit updates isValidProtoForLibFunc() to check that the argument
is a pointer type. This also drops the check for i8* since we should not
be checking the pointee type any more.

Reviewed By: arsenm
Differential Revision: https://reviews.llvm.org/D95142