PR33222: Require the declared return type not the actual return type to

PR33222: Require the declared return type not the actual return type to
match when checking for redeclaration of a function template.

This properly handles differences in deduced return types, particularly
when performing redeclaration checks for a friend function template.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@341778 91177308-0d34-0410-b5e6-96231b3b80d8