[Clang] always_inline statement attribute

Motivation:

```
int test(int x, int y) {
    int r = 0;
    [[clang::always_inline]] r += foo(x, y); // force compiler to inline this function here
    return r;
}
```

In 2018, @kuhar proposed "Introduce per-callsite inline intrinsics" in https://reviews.llvm.org/D51200 to solve this motivation case (and many others).

This patch solves this problem with call site attribute. "noinline" statement attribute already landed in D119061. Also, some LLVM Inliner fixes landed so call site attribute is stronger than function attribute.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D120717