[clangd] Add "clangd.trace" VSCode setting to enable tracing.

Summary:
Setting the CLANGD_TRACE environment variable directly is awkward with VSCode's
"reload from the command palette" workflow.

Reviewers: ilya-biryukov

Subscribers: klimek, jkorous-apple, ioeric, cfe-commits

Differential Revision: https://reviews.llvm.org/D43385

llvm-svn: 325477