This is a fixed version of #106185, which was reverted in #113978 due to
a buildbot failure.
Motivation example:
```
> cat test.cpp
extern "C" [[gnu::weak]] void f() {}
void alias() __attribute__((alias("f")));
int main() { auto p = alias; p(); }
> clang test.cpp -fsanitize=cfi-icall -flto=thin -fuse-ld=lld
> ./a.out
[1] 1868 illegal hardware instruction ./a.out
```
If the address of a function was only taken through its alias, the
function was not considered exported and therefore was not included in
the CFI jumptable. This resulted in `@llvm.type.test()` being lowered to
`false`, and consequently the indirect call to the function was
eventually optimized to `ubsantrap()`.
454abad7b0