Auto merge of #36214 - jseyfried:stackless_expansion, r=nrc

macros: stackless expansion

After this PR, macro expansion cannot overflow the stack unless the expanded crate is too deep to fold.
Everything but the stackless placeholder expansion commit is also groundwork for macro modularization.

r? @nrc or @eddyb

diff --cc src/librustc_driver/driver.rs
Simple merge