Add `gen-type-constraint-decls` and `gen-type-constraint-defs`, which
generate public C++ functions for type constraints. The name of the C++
function is specified in the `cppFunctionName` field.
Type constraints are typically used for op/type/attribute verification.
They are also sometimes called from builders and transformations. Until
now, this required duplicating the check in C++.
Note: This commit just adds the option for type constraints, but
attribute constraints could be supported in the same way.
Alternatives considered:
1. The C++ functions could also be generated as part of
`gen-typedef-decls/defs`, but that can be confusing because type
constraints may rely on type definitions from multiple `.td` files.
`#include`s could cause duplicate definitions of the same type
constraint.
2. The C++ functions could also be generated as static member functions
of dialects, but they don't really belong to a dialect. (Because they
may rely on type definitions from multiple dialects.)
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