[clang-tools-extra] [clangd] Resolve __builtin_offsetof designator components precisely (PR #194407)

[Aleksandr Platonov via cfe-commits]

https://github.com/ArcsinX approved this pull request.

Thanks, LGTM.

Perhaps the description needs to be slightly corrected according to your latest change.

P.S. 
I also noticed that the completion doesn't work (i.e. ctrl+space on ^ doesn't return `field` for code like `struct S { int field; }; int x = __builtin_offsetof(struct S, fiel^d);`). This is definitely outside the scope of the current PR, but you might be interested in looking into it.

https://github.com/llvm/llvm-project/pull/194407