Implement an indirect-goto optimization for goto *&&lbl and respect this

Implement an indirect-goto optimization for goto *&&lbl and respect this
in the scope checker.  With that done, turn an indirect goto into a
protected scope into a hard error;  otherwise IR generation has to start
worrying about declarations not dominating their scopes, as exemplified
in PR8473.

If this really affects anyone, I can probably adjust this to only hard-error
on possible indirect gotos into VLA scopes rather than arbitrary scopes.
But we'll see how people cope with the aggressive change on the marginal
feature.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@117539 91177308-0d34-0410-b5e6-96231b3b80d8