[clang] [Clang] Check enable_if attribute in the DC of current function (PR #175899)

[Arthur Eubanks via cfe-commits]

aeubanks wrote:

I also hit a crash with this, reduced to

```
const int kMaxNumber = 1;
struct Arg {
  Arg(int);
};
void PlaceholderBitmask();
void Substitute(Arg) __attribute__((enable_if(PlaceholderBitmask, ""))) {
  [] { Substitute(kMaxNumber); };
}
```

https://github.com/llvm/llvm-project/pull/175899