6745: Some more proc macro cleanups r=jonas-schievink a=jonas-schievink
* Remove `ProcMacroClient::dummy` and just use `Option<ProcMacroClient>` instead
* Remember the type of proc macros (later allows us to reject using an incorrect macro type)
* Prepare a few internals for procedural attribute macros
bors r+
Co-authored-by: Jonas Schievink <jonasschievink@gmail.com>
c1824b4933