[Parse] Add diagnostic for extraneous case keyword when multiple patterns.

[gregomni]

This change adds a diagnostic for code that tries to use a second case keyword when writing a case statement with multiple patterns. E.g. case .a, case .b: instead of case .a, .b:. I've seen several people do this while pair programming and a recent question in the iOS-dev Slack on how to do this made me realize the current diagnostics for this (IMHO) common error are pretty terrible:

test.swift:8:12: error: expected pattern
  case .a, case .b:
           ^
test.swift:8:12: error: expected ':' after 'case'
  case .a, case .b:
           ^
test.swift:8:17: warning: case is already handled by previous patterns; consider removing it
  case .a, case .b:
                ^~

(This is because the second case causes pattern parsing to fail and the recovery inserts a match-anything pattern - thus the confusing warning.)

With this change, you get a single error, and a fix-it to remove the offending token:

test.swift:8:12: error: extraneous 'case' keyword in pattern

[gregomni]

@swift-ci Please smoke test

[ahoppen]

Thanks you. Looks good. Would you be interested in also fixing this in the new parser?

[gregomni]

Sure, will do, thanks!