Reject jumping inside stmtexprs
Reject jumping inside stmtexprs

One can't jump into statement expressions from outside
them, like the following:

  int i = ({ label: foo(); 42; });
  goto label;

We reject this by making the labels simply not available
outside (GCC has a nicer error message about jumping into
a statement expression).

run4flat/tinycc,code4game/tinycc,run4flat/tinycc,avih/tinycc,run4flat/tinycc,avih/tinycc,mirror/tinycc,code4game/tinycc,mingodad/tinycc,andreiw/tinycc,avih/tinycc,mingodad/tinycc,avih/tinycc,andreiw/tinycc,mingodad/tinycc,run4flat/tinycc,mirror/tinycc,mingodad/tinycc,mirror/tinycc,andreiw/tinycc,mirror/tinycc,run4flat/tinycc,code4game/tinycc