Remove old util/liveness.rs module

[ecstatic-morse]

The liveness dataflow analysis now lives in the dataflow module, so this one is no longer necessary. I've copied the relevant bits of the module docs for util::liveness to MaybeLiveLocals. The example in the docs is now a mir-dataflow test:

rust/src/test/ui/mir-dataflow/liveness-ptr.rs

Lines 6 to 26 in a08c473

	fn foo() -> i32 {
	let mut x: i32;
	let mut p: *const i32;
	x = 0;
	// `x` is live here since it is used in the next statement...
	unsafe { rustc_peek(x); }
	p = &x;
	// ... but not here, even while it can be accessed through `p`.
	unsafe { rustc_peek(x); } //~ ERROR rustc_peek: bit not set
	let tmp = unsafe { *p };
	x = tmp + 1;
	unsafe { rustc_peek(x); }
	x
	}

The borrow-checker used the same notion of "defs" and "uses", so I've moved it into a submodule. I would have moved it to util/def_use.rs, since it seems generally useful, but there's already a slightly different version of the same abstraction needed for copy propagation.

[rust-highfive]

r? @cramertj

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[jonas-schievink]

@bors r+

[bors]

📌 Commit 046848e has been approved by jonas-schievink

[bors]

🌲 The tree is currently closed for pull requests below priority 100, this pull request will be tested once the tree is reopened