[clangd] Add semanticTokens modifiers for function/class/file/global scope

These allow (function-) local variables to be distinguished, but also a
bunch more cases.
It's not quite independent with existing information (e.g. the
field/variable distinction is redundant if you have class-scope + static
attributes) but I don't think this is terribly important.

Depends on D77811

Differential Revision: https://reviews.llvm.org/D95701