Auto merge of #24205 - brson:debug, r=alexcrichton

This makes the default configuration fully optimized, with no debugging options, no llvm asserts, renames --enable-debug to --enable-debug-assertions, and adds --enable-debug as a blanket option that toggles various things, per #17665. It does not add a `--enable-release` flag since that would be a no-op.

cc @nrc

Fixes https://github.com/rust-lang/rust/issues/22390
Fixes https://github.com/rust-lang/rust/issues/17081
Partially addresses https://github.com/rust-lang/rust/issues/17665