[PDLL] Add support for user defined constraint and rewrite functions

These functions allow for defining pattern fragments usable within the `match` and `rewrite` sections of a pattern. The main structure of Constraints and Rewrites functions are the same, and are similar to functions in other languages; they contain a signature (i.e. name, argument list, result list) and a body:

```pdll
// Constraint that takes a value as an input, and produces a value:
Constraint Cst(arg: Value) -> Value { ... }

// Constraint that returns multiple values:
Constraint Cst() -> (result1: Value, result2: ValueRange);
```

When returning multiple results, each result can be optionally be named (the result of a Constraint/Rewrite in the case of multiple results is a tuple).

These body of a Constraint/Rewrite functions can be specified in several ways:

* Externally
In this case we are importing an external function (registered by the user outside of PDLL):

```pdll
Constraint Foo(op: Op);
Rewrite Bar();
```

* In PDLL (using PDLL constructs)
In this case, the body is defined using PDLL constructs:

```pdll
Rewrite BuildFooOp() {
  // The result type of the Rewrite is inferred from the return.
  return op<my_dialect.foo>;
}
// Constraints/Rewrites can also implement a lambda/expression
// body for simple one line bodies.
Rewrite BuildFooOp() => op<my_dialect.foo>;
```

* In PDLL (using a native/C++ code block)
In this case the body is specified using a C++(or potentially other language at some point) code block. When building PDLL in AOT mode this will generate a native constraint/rewrite and register it with the PDL bytecode.

```pdll
Rewrite BuildFooOp() -> Op<my_dialect.foo> [{
  return rewriter.create<my_dialect::FooOp>(...);
}];
```

Differential Revision: https://reviews.llvm.org/D115836

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