[clangd] Expose more dependent-name detail via semanticTokens

This change makes dependentName a modifier, rather than a token type.
It can be combined with:
- type (new, standard) - this combination replaces dependentType like T::typename Foo
- unknown (new, nonstandard) - for general dependent names
- Field, etc - when the name is dependent but we heuristically resolve it

While here, fix cases where template-template-parameter cases were
incorrectly flagged as type-dependent.
And the merging of modifiers when resolving conflicts accidentally
happens to work around a bug that showed up in a test.

The behavior observed through the pre-standard protocol should be mostly
unchanged (it'll see the bugfixes only). This is done in a somehat
fragile way but it's not expected to live long.

Differential Revision: https://reviews.llvm.org/D95706