[clang] Fix crash in __builtin_strncmp and other related builtin functions

The implementation of __builtin_strncmp and other related builtins function use
getExtValue() to evaluate the size argument. This can cause a crash when the
value does not fit into an int64_t value, which is can be expected since the
type of the argument is size_t.

The fix is to switch to using getZExtValue().

This fixes: https://github.com/llvm/llvm-project/issues/64876

Differential Revision: https://reviews.llvm.org/D158557