Rollup merge of #65900 - eddyb:proc-macro-cleanup, r=alexcrichton
Rollup merge of #65900 - eddyb:proc-macro-cleanup, r=alexcrichton

proc_macro: clean up bridge::client::__run_expand{1,2} a bit.

See commit titles/diffs for more details.

The first commit is made possible by #53451 being fixed (almost a year ago).
The last commit should remove the need for `#[allow(improper_ctypes)]` in #65134.

aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust