Auto merge of #38783 - alexcrichton:stabilize-proc-macro, r=nikomatsakis
rustc: Stabilize the `proc_macro` feature
This commit stabilizes the `proc_macro` and `proc_macro_lib` features in the
compiler to stabilize the "Macros 1.1" feature of the language. Many more
details can be found on the tracking issue, #35900.
Closes #35900