Parse: Recover more gracefully from extra :: tokens before a {

Instead of crashing, recover by eating the extra trailing scope
qualifier.  This means we will treat 'struct A:: {' as 'struct A {'.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224966 91177308-0d34-0410-b5e6-96231b3b80d8