[clang-tidy] Allow `TransformerClangTidyCheck` clients to set the rule directly.

Adds support for setting the `Rule` field. In the process, refactors the code that accesses that field and adds a constructor that doesn't require a rule argument.

This feature is needed by checks that must set the rule *after* the check class
is constructed. For example, any check that maintains state to be accessed from
the rule needs this support. Since the object's fields are not initialized when
the superclass constructor is called, they can't be (safely) captured by a rule
passed to the existing constructor.  This patch allows constructing the check
superclass fully before setting the rule.

As a driveby fix, removed the "optional" from the rule, since rules are just a
set of cases, so empty rules are evident.

Differential Revision: https://reviews.llvm.org/D91544