[Parser]: Labeled block without do diagnostics

[kitaisreal]

Improve diagnostics for labeled block without 'do'

More information in ticket https://bugs.swift.org/browse/SR-3867

[varungandhi-apple]

@kitaisreal, going forward, could you add some more information in the commit messages? Right now, the two commit messages in this PR are very similar. There are a couple of options, whichever works for you: (1) make changes as suggested, rebase + force-push (review is still going on) or (2) make changes as suggested in new commits, rebase + force-push after changes are approved, retest and merge.

[kitaisreal]

Hi, @varungandhi-apple lets make changes in new commits after review, then rebase, force push and retest.

[rintaro]

I was thinking of another strategy for this.

• isStartOfStmt() returns true for identifier ':' '{'
• In parseStmt(), add tok::l_brace case to handle identifier ':' '{' case.
• Add an option to parseStmtDo() to indicate consuming do should be skipped.
• In parseStmtDo(), if the flag is true, use the location of { for do keyword location.

Using this strategy, we can stop emitting use of unresolved identifier diagnostics, and parse the body as the body of the 'do' statement (instead of skipping them).
What do you think?

[kitaisreal]

Hi, @rintaro totally agree. Fixed.

[rintaro]

This looks great overall! Just a few comments.

[rintaro]

@swift-ci Please smoke test

[rintaro]

@swift-ci Please smoke test macOS

[rintaro]

Thank you @kitaisreal !