[mlir][sparse] Improve `DimLvlMapParser`'s handling of variable bindings

This commit comprises a number of related changes:

(1) Reintroduces the semantic distinction between `parseVarUsage` vs `parseVarBinding`, adds documentation explaining the distinction, and adds commentary to the one place that violates the desired/intended semantics.

(2) Improves documentation/commentary about the forward-declaration of level-vars, and about the meaning of the `bool` parameter to `parseLvlSpec`.

(2) Removes the `VarEnv::addVars` method, and instead has `DimLvlMapParser` handle the conversion issues directly.  In particular, the parser now stores and maintains the `{dims,lvls}AndSymbols` arrays, thereby avoiding the O(n^2) behavior of scanning through the entire `VarEnv` for each `parse{Dim,Lvl}Spec` call.  Unfortunately there still remains another source of O(n^2) behavior, namely: the `AsmParser::parseAffineExpr` method will copy the `DimLvlMapParser::{dims,lvls}AndSymbols` arrays into `AffineParser::dimsAndSymbols` on each `parse{Dim,Lvl}Spec` call; but fixing that would require extensive changes to `AffineParser` itself.

Depends On D155532

Reviewed By: Peiming

Differential Revision: https://reviews.llvm.org/D155533

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