Fix build break due to signature change on ASTContext' setExternalSource parameter.

This change converts points to clang::ExternalASTSource from llvm::OwningPtr<> to
llvm::IntrusiveRefCntPtr<>.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@202411 91177308-0d34-0410-b5e6-96231b3b80d8