[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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66335

llvm-svn: 369238