Auto merge of #12858 - fasterthanlime:proc-macro-srv-bin, r=Veykril

Add `rust-analyzer-proc-macro-srv` binary, use it if found in sysroot

This adds a `bin` crate which simply runs `proc_macro_srv::cli::run()` (it does no CLI argument parsing, nothing).

The intent is to build that crate in Rust CI as part of the `dist::Rustc` component, then ship it in the sysroot: it would probably land in something like `~/.rustup/toolchains/nightly-2022-07-23-x86_64-unknown-linux-gnu/libexec/proc-macro-srv-cli`.

This makes https://github.com/rust-lang/rustup/pull/3022 less pressing. (Instead of teaching RA about rustup components, we simply teach it to look in the sysroot via `rustc --print sysroot`. If it can't find `proc-macro-srv-cli`, it falls back to its own `proc-macro` subcommand).

This is closely related to https://github.com/rust-lang/rust-analyzer/issues/12803 (but doesn't close it yet).

Things to address now:

  * [ ] What should the binary be named? What should the crate be named? We can pick different names with `[bin]` in the `Cargo.toml`

Things to address later:

  * Disable the "multi ABI compatibility scheme" when building that binary in Rust CI (that'll probably happen in `rust-lang/rust`)
  * Teaching RA to look in the sysroot

Things to address much, much later:

  * Is JSON a good fit here
  * Do we want to add versioning to future-proof it?
  * Other bikesheds

When built with `--features sysroot` on `nightly-2022-07-23-x86_64-unknown-linux-gnu`, the binary is 7.4MB. After stripping debuginfo, it's 2.6MB. When compressed to `.tar.xz`, it's 619KB.

In a Zulip discussion, `@jyn514` and `@Mark-Simulacrum` seemed to think that those sizes weren't a stopper for including the binary in the rustc component, even before we shrink it down further.

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