DebugInfo: don't require full definitions for friend classes

Fixes a crash-on-valid introduced by r188486 (which should've occurred
earlier but for a blatant bug where calling createFwdDecl from the
requireCompleteType callback was useless under -flimit-debug-info and we
were just getting lucky with other later callbacks requiring the type
anyway).

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@188622 91177308-0d34-0410-b5e6-96231b3b80d8