[clangd] allow clients to control the compilation database by passing in

compilationDatabaseChanges in the 'workspace/didChangeConfiguration' request

This commit allows clangd to use an in-memory compilation database that's
controlled from the LSP client (-compile_args_from=lsp). It extends the
'workspace/didChangeConfiguration' request to allow the client to pass in a
compilation database subset that needs to be updated in the workspace.

Differential Revision: https://reviews.llvm.org/D49758

llvm-svn: 338597