[clangd] No need to setTraversalScope in SemanticHighlighting.

Summary:
We have already set it when the AST is being built, and setting TraversalScope
is not free (it will clear the cache, which is expensive to build).

Reviewers: jvikstrom

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63874

llvm-svn: 364528