[mlir][NFC] Use `llvm::to_underlying` in sparse tensor IR detail

[mlir][sparse] Parser cleanup (#69792)

Removed TODOs, FIXMEs and long notes that are more suited for design
doc.

[Support] Remove llvm::is_trivially_{copy/move}_constructible

This restores D132311, which was reverted in
29c841ce93e087fa4e0c5f3abae94edd460bc24a (Sep 2022) due to certain files
not buildable with

[Support] Remove llvm::is_trivially_{copy/move}_constructible

This restores D132311, which was reverted in
29c841ce93e087fa4e0c5f3abae94edd460bc24a (Sep 2022) due to certain files
not buildable with GCC 7.3.0. The previous attempt was reverted by
6cd9608fb37ca2418fb44b57ec955bb5efe10689 (Dec 2020).

This time, GCC 7.3.0 has existing build errors for a long time due to
structured bindings for many files, e.g.

```
llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:9098:13: error: cannot decompose class type ‘std::pair<llvm::Value*, const llvm::SCEV*>’: both it and it
s base class ‘std::pair<llvm::Value*, const llvm::SCEV*>’ have non-static data members
for (auto [_, Stride] : Legal->getLAI()->getSymbolicStrides()) {
^~~~~~~~~~~
```

... and also some `error: duplicate initialization of` instances due to llvm/Transforms/IPO/Attributor.h.

---

GCC 7.5.0 has a bug that, without this change, certain `SmallVector` with a `std::pair` element type like `SmallVector<std::pair<Instruction * const, Info>, 0> X;` lead to spurious

```
/tmp/opt/gcc-7.5.0/include/c++/7.5.0/type_traits:878:48: error: constructor required before non-static data member for ‘...’ has been parsed
```

Switching to std::is_trivially_{copy/move}_constructible fixes the error.

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[mlir][sparse] Start migration to new surface syntax for STEA

We are in the progress of migrating to a much improved surface syntax for the Sparse Tensor Encoding Attribute (STEA).

You can see a pr

[mlir][sparse] Start migration to new surface syntax for STEA

We are in the progress of migrating to a much improved surface syntax for the Sparse Tensor Encoding Attribute (STEA).

You can see a preview of this in the StableHLO RFC at

https://github.com/openxla/stablehlo/blob/main/rfcs/20230210-sparsity.md

//**This design is courtesy Wren Romano.**//

This initial revision
(1) Introduces the first version of a new parser written by Wren Romano
(2) Introduces a simple "migration plan" using NEW_SYNTAX on the STEA, which will allow us to test the new parser with new examples, as well as migrate existing examples over without the need to rewrite them all

This first "drop" merely provides the entry points to parse the new syntax. The parser is still under active development. For example, we need to address the "lookahead" issue when parsing the lvl spec (viz. do we see l0 = d0 or a direct d0). Another larger task is to actually implement "affine" parsing (since the MLIR affine parser is not accessible in other parts of the tree).

EXAMPLE:

Currently, CSR looks like

#CSR = #sparse_tensor.encoding<{
lvlTypes = ["dense","compressed"],
dimToLvl = affine_map<(i,j) -> (i,j)>
}>

but you can "force" the new parser with

#CSR = #sparse_tensor.encoding<{
NEW_SYNTAX =
(d0, d1) -> (l0 = d0 : dense, l1 = d1 : compressed)
}>

Reviewed By: Peiming

Differential Revision: https://reviews.llvm.org/D153997

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