Modifies `#llvm.target<..., features = $FEATURES>` so that `$FEATURES`
is now an `#llvm.target_features<[...]>` attribute (rather than a
`StringAttr`). This enables the attribute to respond to DLTI queries for
the different target features.
The pass updates the `$FEATURES` attribute of the target attr at name
`llvm.target` in accordance with the (Sub)Target's features that the
relevant LLVM backend knows about.
---
DEMO:
```mlir
module attributes {llvm.target = #llvm.target<triple = "x86_64-unknown-linux",
chip = "skylake"> } {
}
```
by way of `-llvm-target-to-target-features` turns into:
```mlir
module attributes {llvm.target = #llvm.target<triple = "x86_64-unknown-linux",
chip = "skylake",
features = <["+64bit", "+64bit-mode", "+adx", "+aes", "+allow-light-256-bit", "+avx", "+avx2", "+bmi", "+bmi2", "+clflushopt", "+cmov", "+crc32", "+cx16", "+cx8", "+ermsb", "+f16c", "+false-deps-popcnt", "+fast-15bytenop", "+fast-gather", "+fast-scalar-fsqrt", "+fast-shld-rotate", "+fast-variable-crosslane-shuffle", "+fast-variable-perlane-shuffle", "+fast-vector-fsqrt", "+fma", "+fsgsbase", "+fxsr", "+idivq-to-divl", "+invpcid", "+lzcnt", "+macrofusion", "+mmx", "+movbe", "+no-bypass-delay-blend", "+no-bypass-delay-mov", "+no-bypass-delay-shuffle", "+nopl", "+pclmul", "+popcnt", "+prfchw", "+rdrnd", "+rdseed", "+sahf", "+slow-3ops-lea", "+sse", "+sse2", "+sse3", "+sse4.1", "+sse4.2", "+ssse3", "+vzeroupper", "+x87", "+xsave", "+xsavec", "+xsaveopt", "+xsaves"]>>} {
}
```
Adds the `#llvm.target<triple = $TRIPLE, chip = $CHIP, features =
$FEATURES>` attribute and along with a `-llvm-target-to-data-layout`
pass to derive a MLIR data layout from the LLVM data layout string
(using the existing `DataLayoutImporter`). The attribute implements the
relevant DLTI-interfaces, to expose the `triple`, `chip` (AKA `cpu`) and
`features` on `#llvm.target` and the full `DataLayoutSpecInterface`. The
pass combines the generated `#dlti.dl_spec` with an existing `dl_spec`
in case one is already present, e.g. a `dl_spec` which is there to
specify size of the `index` type.
Adds a `TargetAttrInterface` which can be implemented by all attributes
representing LLVM targets.
Similar to the Draft PR https://github.com/llvm/llvm-project/pull/78073.
RFC on which this PR is based:
https://discourse.llvm.org/t/mandatory-data-layout-in-the-llvm-dialect/85875
