Better way to fix bug#70036

Better way to fix bug#70036

Cache a new eglot--cached-tdi var per buffer, which contains value to
return from eglot--TextDocumentIdentifier.

This avoids frequent expensive recomputation of a value that requires
potentially many 'file-truename' calls.  This technique is used in a
number of other cases already, like eglot--recent-changes or
eglot--versioned-identifier.

* lisp/progmodes/eglot.el (eglot--cached-tdi): New variable.
(eglot--TextDocumentIdentifier): Tweak.
(eglot--signal-textDocument/didOpen): Clear eglot--cached-tdi.