Factor out renaming logic from readability-identifier-naming

Before this patch, readability-identifier-naming contained a significant amount
of logic for (a) checking the style of identifiers, followed by (b) renaming/
applying fix-its. This patch factors out (b) into a separate base class so that
it can be reused by other checks that want to do renaming. This also cleans up
readability-identifier-naming significantly, since now it only needs to be
concerned with the interesting details of (a).