rdar://problem/10501020

ClangASTSource::~ClangASTSource() was calling

    ClangASTContext *scratch_clang_ast_context = m_target->GetScratchClangASTContext();

which had the side effect of deleting this very ClangASTSource instance.  Not good.
Change it to

    // We are in the process of destruction, don't create clang ast context on demand
    // by passing false to Target::GetScratchClangASTContext(create_on_demand).
    ClangASTContext *scratch_clang_ast_context = m_target->GetScratchClangASTContext(false);

The Target::GetScratchClangASTContext(bool create_on_demand=true) has a new signature.

llvm-svn: 145537