Check "late parsed" friend functions for redefinition

Check "late parsed" friend functions for redefinition

r177003 applied the late parsed template technique to friend functions
but omitted the corresponding check for redefinitions.

This patch adds the same check already in use for templates to the
new code path in order to diagnose and reject invalid redefinitions
that were being silently accepted.

Fixes PR17324.

Reviewed by Richard Smith.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192948 91177308-0d34-0410-b5e6-96231b3b80d8