Don't assume friended C++ method decls have qualifiers

There are a few cases where unqualified lookup can find C++ methods.
Unfortunately, none of them seem to have illegal access paths, so I
can't excercise the diagnostic source range code that I am changing
here.

Fixes PR21851, which was a crash on valid.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224471 91177308-0d34-0410-b5e6-96231b3b80d8