Add a convenience interface for checking expression arguments to unary

Add a convenience interface for checking expression arguments to unary
traits which uses the information embedded in the expression. Use this
to simplify several interfaces which repeated information embedded in
the expression through explicit arguments. I added an assertion that the
only extra piece of data to come in from the parser matches what is
stored in the expression. No functionality change intended here.

Also cleaned up the doxygen comments for some of these methods and some
formatting oddities.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@132115 91177308-0d34-0410-b5e6-96231b3b80d8