[libTooling] Cleanup and reorder `RewriteRule.h`.

This patch lifts `RootID` out of the `RewriteRule` class so that constructs
(e.g. inline functions) can that refer to the root id don't need to depend on
the `RewriteRule` class.

With this dependency, the patch is able to collect all `ASTEdit` helper function
declarations together with the class declaration, before the introduction of the
`RewriteRule` class. In the process, we also adjust some of the comments.

This patch is essentially a NFC.

Reviewed By: gribozavr2

Differential Revision: https://reviews.llvm.org/D85733