[clang-tidy] Improve "common type" diagnostic output in 'bugprone-easily-swappable-parameters'

Make the check handle cases of the "common type" involved in the mix
being non-trivial, e.g. pointers, references, attributes, these things
coming from typedefs, etc.

This results in clearer diagnostics that have more coverage in their
explanation, such as saying `const int &` as common type instead of
`int`.

Reviewed By: aaron.ballman

Differential Revision: http://reviews.llvm.org/D106442