PR49585: Emit the jump destination for a for loop 'continue' from within the...

PR49585: Emit the jump destination for a for loop 'continue' from within the scope of the condition variable.

The condition variable is in scope in the loop increment, so we need to
emit the jump destination from wthin the scope of the condition
variable.

For GCC compatibility (and compatibility with real-world 'FOR_EACH'
macros), 'continue' is permitted in a statement expression within the
condition of a for loop, though, so there are two cases here:

* If the for loop has no condition variable, we can emit the jump
  destination before emitting the condition.

* If the for loop has a condition variable, we must defer emitting the
  jump destination until after emitting the variable. We diagnose a
  'continue' appearing in the initializer of the condition variable,
  because it would jump past the initializer into the scope of that
  variable.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D98816