cpp11-migrate: Reduce the number of parsing passes.

Previously we would check the syntax of the file before we transform
it, but that's redundant since it'll be checked as part of the
transformation. Remove that check completely.

We also had an unconditional syntax check after transforming. This
is only really useful to debug cpp11-migrate, since users will end
up compiling the transformed source anyways, and the transformations
*should* never introduce a failure. Made this an option, accessible
via "-final-syntax-check".

Resolves PR 15380.
Rewrite a test to check actual output rather than intermediate implementation
detail.

The was this test was written, it was relying on an implementation detail
(fixups) and hence was very brittle (relying, among other things, on the
exact ordering of statistics printed by MC).

The test was rewritten to check a more observable output difference. While it
doesn't cover 100% of the things the original test covered, it's a good
practice to write regression tests this way. If we want to check that
internal details and invariants hold, such tests should be expressed as unit
tests.