Auto merge of #50453 - alexcrichton:proc-macro-not-send, r=eddyb
Auto merge of #50453 - alexcrichton:proc-macro-not-send, r=eddyb

proc_macro: Explicitly make everything !Send/Sync

This commit adds explicit imp blocks to ensure that all publicly exported types
(except simple enums) are not `Send` nor `Sync` in the `proc_macro` crate.

cc #38356

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