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.

llvm-svn: 176376