Fix joaotavora/eglot#116, joaotavora/eglot#150: don't break in indire…

…ct buffers

Indirect buffers, such as the ones created by ediff-regions-wordwise,
have eglot enabled but not buffer-file-name.  Resort to the finding
the file-name of the original buffer for these.

* eglot.el (eglot--TextDocumentIdentifier): Work in indirect
buffers.

eglot.el

@@ -1345,7 +1345,10 @@ THINGS are either registrations or unregisterations."
 
 (defun eglot--TextDocumentIdentifier ()
   "Compute TextDocumentIdentifier object for current buffer."
-  `(:uri ,(eglot--path-to-uri buffer-file-name)))
+  `(:uri ,(eglot--path-to-uri (or buffer-file-name
+                                  (ignore-errors
+                                    (buffer-file-name
+                                     (buffer-base-buffer)))))))
 
 (defvar-local eglot--versioned-identifier 0)