[clangd] Add support for FixIts.
This uses CodeActions to show 'apply fix' actions when code actions are
requested for a location. The actions themselves make use of a
clangd.applyFix command which has to be implemented on the editor side. I
included an implementation for vscode.
This also adds a -run-synchronously flag which runs everything on the main
thread. This is useful for testing.
Differential Revision: https://reviews.llvm.org/D30498