Document the let this = self; idiom used in MIR building

[Zalathar]

In rustc_mir_build there are a few Builder methods that start with let this = self;, so that subsequent code can uniformly refer to the builder as this, instead of having to choose between self at the top level or this when nested in closures that need to borrow the builder.

There is some existing documentation of the idiom in expr_into_dest:

rust/compiler/rustc_mir_build/src/builder/expr/into.rs

Lines 32 to 35 in 69d4d5f

	// since we frequently have to reference `self` from within a
	// closure, where `self` would be shadowed, it's easier to
	// just use the name `this` uniformly
	let this = self;

But that documentation is brief and hard to find, especially if one is unaware that such documentation even exists.

---

This PR therefore adds a longer explanation of the let this = self; idiom in the module documentation for rustc_mir_build::builder, and makes that documentation easier to find by adding a searchable tag (“LET_THIS_SELF”) to the documentation and to each occurrence of the idiom.

[rustbot]

Some changes occurred in match lowering

cc @Nadrieril

[rustbot]

r? @SparrowLii

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[Zalathar]

The lack of documentation has caused confusion in #148955, for example.

[Kivooeo]

@bors r+ rollup

[bors]

📌 Commit 34cccb2 has been approved by Kivooeo

It is now in the queue for this repository.