[clangd] Added highlighting to enum constants.

Summary: VSCode does not have a scope for enum constants. So they were placed under "constant.other.enum" as that seems to be the most correct scope for enum constants. However, this makes theia color them blue (the same color it uses for keywords).

Reviewers: hokein, sammccall, ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64624

llvm-svn: 366045