[clang][Frontend] Add overload to ASTPrinter that doesn't own output stream (#142163)
We're planning on using the ASTPrinter in LLDB for AST dumping. But it
currently takes the output stream via `unique_ptr`. In LLDB we don't
have the output stream available in this form and instead it would be
convenient if we could just pass a reference to the stream.
This patch adds that overload.