This patch adds the `ConvertToLLVMAttrInterface` and
`ConvertToLLVMOpInterface` interfaces. It also modifies the
`convert-to-llvm` pass to use these interfaces when available.
The `ConvertToLLVMAttrInterface` interfaces allows attributes to
configure conversion to LLVM, including the conversion target, LLVM type
converter, and populating conversion patterns. See the `NVVMTargetAttr`
implementation of this interface for an example of how this interface
can be used to configure conversion to LLVM.
The `ConvertToLLVMOpInterface` interface collects all convert to LLVM
attributes stored in an operation.
Finally, the `convert-to-llvm` pass was modified to use these interfaces
when available. This allows applying `convert-to-llvm` to GPU modules
and letting the `NVVMTargetAttr` decide which patterns to populate.
7498eaa9ab