Rollup merge of #67538 - varkor:lhs-assign-diagnostics, r=Centril

Improve diagnostics for invalid assignment

- Improve wording and span information for invalid assignment diagnostics.
- Link to https://github.com/rust-lang/rfcs/issues/372 when it appears the user is trying a destructuring assignment.
- Make the equality constraint in `where` clauses error consistent with the invalid assignment error.

diff --cc src/librustc/hir/mod.rs
Simple merge

diff --cc src/librustc_typeck/check/demand.rs
Simple merge