[clang] [LifetimeSafety] Cache lifetimebound macro lookup. NFC. (PR #205250)

[via cfe-commits]

https://github.com/NeKon69 commented:

I am thinking that you should still add tests for that with a macro redecl chain:
```cpp
#define MACRO [[clang::lifetimebound]]
...
#undef MACRO
...
```

https://github.com/llvm/llvm-project/pull/205250