Simplify and correct the check for function redefinitions. This does two things:

Simplify and correct the check for function redefinitions. This does two things:

  - Allows definitions of overloaded functions :)
  - Eliminates extraneous error messages when we have a definition of a
    function that isn't an overload but doesn't have exactly the same type
    as the original.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@58382 91177308-0d34-0410-b5e6-96231b3b80d8