[Sema] Avoid forming double braced single expression closures
For a single expression closure, just use the
expression as the body in the case where we're
coercing to Void, as the return is already
implied. This avoids crashing in
`ClosureExpr::getSingleExpressionBody` with a
double braced body.
Surprisingly it seems nothing is currently calling
`ClosureExpr::getSingleExpressionBody` after
type-checking, so no test case, but later commits
in this patch will exercise this case.
