[clangd] Fix junk output in clangd vscode plugin

Summary:
When using the vscode clangd plugin, lots and lots of junk output is printed to the output window, which constantly reopens itself.
Example output:

I[11:13:17.733] <-- textDocument/codeAction(4)
I[11:13:17.733] --> reply:textDocument/codeAction(4) 0 ms
I[11:13:17.937] <-- textDocument/codeAction(5)
I[11:13:17.937] --> reply:textDocument/codeAction(5) 0 ms
I[11:13:18.557] <-- textDocument/hover(6)
I[11:13:18.606] --> reply:textDocument/hover(6) 48 ms

This should prevent that from happening.

Patch by James Findley!

Reviewers: ioeric, ilya-biryukov, hokein

Reviewed By: ioeric

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

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D55052

llvm-svn: 347968