[Core] Generalize ValueObject::MaybeCalculateCompleteType

Summary:
Instead of hardcoding ClangASTContext and ObjCLanguageRuntime, we can
generalize this by creating the method GetRuntimeType in
LanguageRuntime and moving the current MaybeCalculateCompleteType
implementation into ObjCLanguageruntime::GetRuntimeType

Reviewers: jingham, clayborg, JDevlieghere

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D64159

llvm-svn: 365939