The idea is to use TableGen records for both custom type constraints and
attributes:
* `PythonTypeName` is for type constraints, while
* `PythonAttrType` is for attributes.
The key types differ between these two records. `PythonTypeName` is
keyed by C++ type because multiple type constraints map to the same C++
type (e.g. `I32` and `I64` both map to `::mlir::IntegerType`), so a
single entry covers all of them. `PythonAttrType` is keyed by TableGen
def name because different attributes can share the same C++ storage
type but need distinct Python types (e.g. `I32ArrayAttr` and
`StrArrayAttr` are both `::mlir::ArrayAttr`).
We could in theory reimplement `getPythonAttrName` using the same
approach, but I decided to leave it for future PRs.
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