Fix an issue uncovered by the stress tester where
the brace element skipping logic could still attempt
to skip a single-expression body of an if/switch
expr.
Skip type-checking multi-statement branches if the
completion is in a single-expression branch, and
skip type-checking the expression as a whole if
the completion is in a multi-statement branch.
Run PreCheckFunctionBodyRequest to ensure we insert
an implicit return for an if/switch if needed, and
ensure we don't try and type-check an element in a
SingleValueStmtExpr separately, as it should be
type-checked as a whole by the constraint system.
This ensures we can propagate a contextual type from
outside an if/switch expression for code completion.
