swift234

[CodeCompletion] Improve StmtCondition recovery for code-completion

In the condition expressions of if, while and guard statements we were
throwing away the AST if there was a parse error in the condition, or
the brace statement was missing.  This led to poor code-completion for
unresolved members (enums, options sets) because we couldn't find the
parent expression to type-check.

There are a few minor diagnostic changes because we now do more
type-checking of these conditions, particularly if they end up
containing an unused closure.

SR-2001