[swift-4.0-branch] [objc] Don't emit memset when result is ignored for struct-returning method calls (#99)

This is a cherry-pick/back-port of a part of r306838, "[objc] Don't require null-check and don't emit memset when result is ignored for struct-returning method calls". Fixes an issue with the emission of lifetime markers for struct-returning Obj-C msgSend calls.
2 files changed