This PR replaces the catch-all "unsupported pattern in syntax match"
error that the new `do` elaborator produces for typical pattern mistakes
(#2215, #8304, #10393) with the proper diagnostics from the regular
pattern-var collector (e.g. "Invalid pattern: Expected a constructor or
constant marked with `[match_pattern]`", "ambiguous pattern, use fully
qualified name"), pointing at the offending pattern.
`getPatternVarsEx` / `getPatternsVarsEx` in `Lean.Elab.Do` now try the
syntax-quotation collector first (cheaply handling identifiers, holes,
and antiquotations) and fall back to the regular pattern-var collector
for everything else. When both fail, the regular collector's error wins
via `<|>` semantics.
The legacy `do` elaborator is intentionally left untouched, so the
existing `tests/elab/doSyntaxPatternError.lean` guards (which capture
the cryptic messages produced under legacy default) are unchanged. They
will need updating when the new `do` elaborator becomes default.
Fixes#2215, #8304, and #10393 for the new `do` elaborator.
