Diagnose friend function template redefinitions.

Diagnose friend function template redefinitions.

Friend function template defined in a class template becomes available if
the enclosing class template is instantiated. Until the function template
is used, it does not have a body, but still is considered a definition for
the purpose of redeclaration checks.

This change modifies redefinition check so that it can find the friend
function template definitions in instantiated classes.

Differential Revision: http://reviews.llvm.org/D21508

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@348473 91177308-0d34-0410-b5e6-96231b3b80d8