[clangd] Added highlighting for constructor initializers.

Summary: Constructor initializers were not being highlighted. This adds highlighting for them by using TraverseConstructorInitializer. Uses the Traverse* because there is no visit for CXXCtorInitializer.

Reviewers: hokein, ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66001

llvm-svn: 368434