Add proc-macro-srv integration test that clones literals
This exercises some of the upcoming proc_macro bridge changes. It should also pass for all supported ABIs, with the older-style bridge.
This changed is tracked in:
* https://github.com/rust-lang/rust-analyzer/issues/12818
Building it in-place fails in rust CI because the source directory
is read-only. This changes `proc-macro-test`'s build script to first
copy `imp` under `OUT_DIR` (which is read-write).
It also prints stdout/stderr for the nested cargo invocation, should
it fail. (I've seen failures in rust CI that I couldn't explain, and
when they take 25 minutes to reproduce, you want to have that info)
