[clang-tidy] Fix crash on "reference-to-array" parameters in 'bugprone-easily-swappable-parameters'

An otherwise unexercised code path related to trying to model
"array-to-pointer decay" resulted in a null pointer dereference crash
when parameters of type "reference to array" were encountered.

Fixes crash report http://bugs.llvm.org/show_bug.cgi?id=50995.

Reviewed By: aaron.ballman

Differential Revision: http://reviews.llvm.org/D106946