deductions of the same template parameter are equivalent. This allows
us to implement the is_same type trait (!).

Also, move template argument deduction into its own file and update a
few build systems with this change (grrrr).



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@72819 91177308-0d34-0410-b5e6-96231b3b80d8