[clang] Enable unguarded availability diagnostic on instantiated template functions (PR #91699)

[Amy Huang via cfe-commits]

amykhuang wrote:

here's a reduced example of what we ran into in Chrome. I guess the attribute is being applied to a function that's passed to `f()`, and the warning points to the call in the definition of `f`. 

```
#define a(b) __attribute__((__availability__(android, introduced = b)))
#define c(b) (a(b), apply_to = function)

template <typename callback> void f(callback cb) {
  cb();
}

#pragma clang attribute push c(29)
void test() {
  f([](){});
}
#pragma clang attribute pop
```

https://github.com/llvm/llvm-project/pull/91699