[UTC] Include return type/attributes under --version 2

If --function-signature is used with --version 2, then also include
the return type/attributes in the check lines. This is the
implementation of D133943 rebased on the --version mechanism from
D142473.

This doesn't bump the default version yet, because I'd like to do
that together with D140212 (which enables --function-signature by
default), as these changes seem closely related. For now this
functionality can be accessed by explicitly passing --version 2
to UTC.

Fixes https://github.com/llvm/llvm-project/issues/61058.

Differential Revision: https://reviews.llvm.org/D144963