[clangd] Handle clangd.applyFix server-side

Summary:
When the user selects a fix-it (or any code action with commands), it is
possible to let the client forward the selected command to the server.
When the clangd.applyFix command is handled on the server, it can send a
workspace/applyEdit request to the client. This has the advantage that
the client doesn't explicitly have to know how to handle
clangd.applyFix. Therefore, the code to handle clangd.applyFix in the VS
Code extension (and any other Clangd client) is not required anymore.

Reviewers: ilya-biryukov, sammccall, Nebiroth, hokein

Reviewed By: hokein

Subscribers: ioeric, hokein, rwols, puremourning, bkramer, ilya-biryukov

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D39276

llvm-svn: 317322