Add initial support for NonNull attribute.
(https://github.com/llvm/llvm-project/issues/57113)
Test plan:
verify that for
__thread int x;
int main() {
int* y = &x;
return *y;
}
(with this patch) clang -O -fsanitize=null -S -emit-llvm -o -
doesn't emit a null-pointer check
Differential revision: https://reviews.llvm.org/D131872
d68ba43ad2