[Target] Hoist LanguageRuntime::GetDeclVendor
It's possible that each LanguageRuntime could have its own DeclVendor,
so let's hoist that out of ObjCLanguageRuntime into LanguageRuntime.
Additionally, this gives the opportunity to remove SBTarget's dependency
Reviewers: JDevlieghere, labath, compnerd, davide
Differential Revision: https://reviews.llvm.org/D63622