Tweak the assert in ModuleBuilder from r224533 (PR21989)

Turns out there will be left-over deferred inline methods if there have
been errors, because in that case HandleTopLevelDecl bails out early.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224649 91177308-0d34-0410-b5e6-96231b3b80d8