Stop diagnosing the use of inner classes as friends. ddunbar asked whether

Stop diagnosing the use of inner classes as friends.  ddunbar asked whether
this was useful, and on review Doug and I decided it was probably on the level
of a bug in the standard and therefore not worth a warning even in -pedantic.
If someone disagrees and urgently wants clang++ to warn about this in strict
c++98 mode, we can talk about it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@91868 91177308-0d34-0410-b5e6-96231b3b80d8