Auto merge of #49219 - eddyb:proc-macro-decouple, r=alexcrichton

Decouple proc_macro from the rest of the compiler.

This PR removes all dependencies of `proc_macro` on compiler crates and allows multiple copies of `proc_macro`, built even by different compilers (but from the same source), to interoperate.

Practically, it allows:
* running proc macro tests at stage1 (I moved most from `-fulldeps` to the regular suites)
* using proc macros in the compiler itself (may require some rustbuild trickery)

On the server (i.e. compiler front-end) side:
* `server::*` traits are implemented to provide the concrete types and methods
  * the concrete types are completely separated from the `proc_macro` public API
  * the only use of the type implementing `Server` is to be passed to `Client::run`

On the client (i.e. proc macro) side (potentially using a different `proc_macro` instance!):
* `client::Client` wraps around client-side (expansion) function pointers
  * it encapsulates the `proc_macro` instance used by the client
  * its `run` method can be called by a server, to execute the client-side function
    * the client instance is bridged to the provided server, while it runs
    * ~~currently a thread is spawned, could use process isolation in the future~~
(not the case anymore, see #56058)
* proc macro crates get a generated `static` holding a `&[ProcMacro]`
  * this describes all derives/attr/bang proc macros, replacing the "registrar" function
  * each variant of `ProcMacro` contains an appropriately typed `Client<fn(...) -> ...>`

`proc_macro` public APIs call into the server via an internal "bridge":
* only a currently running proc macro `Client` can interact with those APIs
  * server code might not be able to (if it uses a different `proc_macro` instance)
    * however, it can always create and `run` its own `Client`, but that may be inefficient
* the `bridge` uses serialization, C ABI and integer handles to avoid Rust ABI instability
* each invocation of a proc macro results in disjoint integers in its `proc_macro` handles
  * this prevents using values of those types across invocations (if they even can be kept)

r? @alexcrichton cc @jseyfried @nikomatsakis @Zoxc @thepowersgang

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