[Target] Hoist LanguageRuntime::GetDeclVendor

Summary:
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
on ObjCLanguageRuntime.

Reviewers: JDevlieghere, labath, compnerd, davide

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D63622

llvm-svn: 364229