[clangd] Added special HighlightingKind for function parameters.
Summary: This means that function parameters are no longer highlighted as variable.other.cpp but instead as variable.parameter.cpp which is the more "correct" TextMate scope for them.
Reviewers: hokein, ilya-biryukov
Subscribers: MaskRay, jkorous, arphaman, kadircet, cfe-commits
Differential Revision: https://reviews.llvm.org/D66335