Implement jump checking for initialized c++ variables, implementing

Implement jump checking for initialized c++ variables, implementing
a fixme and PR6451.

Only perform jump checking if the containing function has no errors,
and add the infrastructure needed to do this.

On the testcase in the PR, we produce:

t.cc:6:3: error: illegal goto into protected scope
  goto later;
  ^
t.cc:7:5: note: jump bypasses variable initialization
  X x;
    ^

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@97497 91177308-0d34-0410-b5e6-96231b3b80d8