Permit a class to friend its members in C++0x, without restriction.

Permit a class to friend its members in C++0x, without restriction.
Permit a class to friend its class members in C++ 98, as long as extensions
are enabled (and even when they aren't, only give an extwarn about it).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@78332 91177308-0d34-0410-b5e6-96231b3b80d8