Move friend warning into CXX11 warning group.

Also, fix the wording to reflect this.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187171 91177308-0d34-0410-b5e6-96231b3b80d8
Partially revert r186903.

It turns out that Plum Hall depends on us not emitting an error on
integer literals which fit into long long, but fit into
unsigned long long.  So C99 conformance requires not conforming to C99. :)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187172 91177308-0d34-0410-b5e6-96231b3b80d8