Make trans treat all statements as if they introduce a new scope

[catamorphism]

r? @graydon This is for consistency with borrowck. Changing borrowck would
also be an alternative, but I found it easier to change trans :-)

This eliminates an ICE in trans where the scope for a particular
borrow was a statement ID, but the code in trans that does cleanups
wasn't finding the block with that scope. As per #3860

[graydon]

r+ with a couple caveats:

• it shouldn't cause anything else to break: I'm assuming this scope isn't going to accidentally cause stuff to be freed early or something? A trans scope isn't exactly a {..} lifetime-of-a-variable thing, right?
• check that it's not a huge perf regression to be adding all these scopes. perf stat before-and-after runs to see.
• enhance the comment in trans_stmt here so it points to region::resolve_stmt, and add a comment there pointing back to trans::base::trans_stmt, so that both sites indicate their inter-relatedness.

Sorry, this stuff is delicate-city and I know we're likely to revisit it. Best to lay down invariant-describing docs as we go.

[catamorphism]

1. No, because this is always adding a larger scope as a super-scope of a scope that would have existed before. If that's clear.
2. I'll try that
3. Ditto

[catamorphism]

Instruments.app reported a total 13m:30s build time for a clean build of make perf with the incoming branch, and 15m:30s with this change. (With optimization enabled.) That seems pretty bad, so I'll think about other ways to accomplish this.

[catamorphism]

I'm going to close this since it's pretty clearly not the right solution. The bug is still open.