compiler: correct parse of parenthesized select case

We used to mishandle `select { case (<-c): }` and friends.

The test case for this is https://golang.org/cl/91657.

Fixes golang/go#20923

Change-Id: I31a2afae3728f70fc7b788d7b9661579dcc401ce
Reviewed-on: https://go-review.googlesource.com/91695
Reviewed-by: Than McIntosh <thanm@google.com>
1 file changed