[clangd] Add 'Switch header/source' command in clangd-vscode

Summary:
Alt+o is used on Windows/Linux and Option+Cmd+o on macOS.

Signed-off-by: Marc-Andre Laperle <malaperle@gmail.com>

Reviewers: hokein, ilya-biryukov, ioeric

Reviewed By: ioeric

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

Differential Revision: https://reviews.llvm.org/D54781

llvm-svn: 347509