[region-isolation] Simplify the code for use after transfer errors.

Specifically, we previously would generate a list of errors to emit and then
have one centralized piece of code that went through the list and emitted them.
Instead, we now have a low level emitter struct that is just used to emit the
diagnostic at the immediate point where we would create previously an error
record. This places the diagnostic emission next to the piece of code that
triggered it being emitted making it easier to debug why an error was emitted
using -Xllvm -swift-diagnostics-assert-on-*.

I am going to do the same for transfer non transferrable in a little bit once
the async let patch has landed. It has some transfer non transferable error
additions that I want to get in so I am going to wait on that.

NFCI.

Author: gottesmm