gnu::always_inline functions, which lower to available_externally, may
not have definitions external to the module. -finstrument-function
family options instrumentating the function (which takes the function
address) may lead to a linker error if the function is not optimized
out, e.g.
```
// -std=c++17 or above with libstdc++
#include <string>
std::string str;
int main() {}
```
Simplified reproduce:
```
template <typename T>
struct A {
[[gnu::always_inline]] T bar(T a) { return a * 2; }
};
extern template class A<int>;
int main(int argc, char **argv) {
return A<int>().bar(argc);
}
```
GCC's -finstrument-function instrumentation skips such functions
(https://gcc.gnu.org/PR78333). Let's skip such functions
(available_externally) as well.
Fix#50742
Pull Request: https://github.com/llvm/llvm-project/pull/121452
e6f76378c2