Add `Context::request_discard` for multi-pass layouts #12699

Job	Run time
Check wasm32 + wasm-bindgen	2m 22s
Format + check + test	5m 21s
android	23s
ios	22s
cargo-deny aarch64-apple-darwin	41s
Check Windows	1m 23s
cargo-deny aarch64-linux-android	40s
cargo-deny i686-pc-windows-gnu	39s
cargo-deny i686-pc-windows-msvc	39s
cargo-deny i686-unknown-linux-gnu	37s
cargo-deny wasm32-unknown-unknown	38s
cargo-deny x86_64-apple-darwin	40s
cargo-deny x86_64-pc-windows-gnu	38s
cargo-deny x86_64-pc-windows-msvc	37s
cargo-deny x86_64-unknown-linux-gnu	38s
cargo-deny x86_64-unknown-linux-musl	36s
cargo-deny x86_64-unknown-redox	37s
	17m 31s

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