Using Emacs 27 on Linux, prior to this commit, when clicking on the 'N refs' annotation created by
code lens, you would get "cl--assertion-failed: Assertion failed: (functionp fetcher)".

The issue is in ccls.el, lsp-execute-command which was calling xref--show-xrefs when it should
have been calling lsp-show-xrefs. I also fixed a similar issue in ccls-code-lens.el.

See emacs-lsp/lsp-mode#3838

Prior to this fix, when using ccls-member-hierarchy, followed by a mouse-1 clicking in the member
hierarchy view, you'll see:

     Debugger entered--Lisp error: (void-variable cur-buffer-workspaces)
       ccls-tree-press()
       ccls-tree-press-and-switch()
       funcall-interactively(ccls-tree-press-and-switch)
       call-interactively(ccls-tree-press-and-switch nil nil)
       command-execute(ccls-tree-press-and-switch)

There's still two left (on Emacs 27):

    ccls-inheritance-hierarchy.el:49:1:Warning: Unused lexical variable ‘children’
    ccls.el:153:27:Warning: Unused lexical argument ‘command’

Not sure what the right fix is for these (is it incomplete code?)