rewrite the goto scope checking code to be more efficient, simpler,

produce better diagnostics, and be more correct in ObjC cases (fixing
rdar://6803963).

An example is that we now diagnose:

int test1(int x) {
  goto L;
  int a[x];
  int b[x];
  L:
  return sizeof a;
}

with:

scope-check.c:15:3: error: illegal goto into protected scope
  goto L;
  ^
scope-check.c:17:7: note: scope created by variable length array
  int b[x];
      ^
scope-check.c:16:7: note: scope created by variable length array
  int a[x];
      ^

instead of just saying "invalid jump".  An ObjC example is:

void test1() {
  goto L;
  @try {
L: ;
  } @finally {
  }
}

t.m:6:3: error: illegal goto into protected scope
  goto L;
  ^
t.m:7:3: note: scope created by @try block
  @try {
  ^

There are a whole ton of fixme's for stuff to do, but I believe that this
is a monotonic improvement over what we had.

llvm-svn: 69437