ModuleBuilder: assert that all deferred inline method defs get handled

While we're here, also move the declaration of DeferredInlineMethodDefinitions
closer to the other member vars and make it a SmallVector. NFC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224533 91177308-0d34-0410-b5e6-96231b3b80d8