[clangd][vscode] Make SemanticHighlightingFeature more self-contained.

Summary:
so that we don't have too many usage from the client side (just a single
occurrance for register), this also aligns with how other builtin feature
being implemented in vscode.

Reviewers: ilya-biryukov

Subscribers: MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67165

llvm-svn: 371036