[llvm] [Dexter] Add basic structured script parsing (PR #193710)

[Orlando Cazalet-Hyams via llvm-commits]

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+# DExTer : Debugging Experience Tester
+# ~~~~~~   ~         ~~         ~   ~~
+#
+# Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+# See https://llvm.org/LICENSE.txt for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+"""This file defines all of the Nodes used to create Dexter scripts. All Nodes must be registered with the yaml
+constructor/representer in `setup_yaml_parser` before loading or printing any script.
+"""
+
+import abc
+from dataclasses import dataclass
+from typing import Any, Dict, Optional, Union
+import yaml
+from dex.dextIR.ValueIR import ValueIR
+from dex.utils.Exceptions import Error
+
+
+def setup_yaml_parser(loader):
+    reg_classes = [
+        Where,
+        Value,
+        DexRange,
+    ]
+    for c in reg_classes:
+        c.register_yaml(loader)
+
+
+class DexterNodeError(Error):
+    """Class representing errors with Dexter node parsing."""
+
+    def __init__(self, node, msg):
+        super(DexterNodeError, self).__init__(msg)
+        self.msg = msg
+        self.node = node
+
+    def __str__(self):
+        return f"Error with node: {self.node}: {self.msg}"
+
+
+###################
+## Structural Nodes: These are used as keys in the Script, and collectively define Dexter's actions when running a test:
+##                   how it steps and navigates through the debuggee program, and what information it collects from the
+##                   debugger.
+
+
+class Where:
+    """ "One or more instances of this class define a range of steps in a debugging session. Any expects in the script
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OCHyams wrote:

nit, extra "?

https://github.com/llvm/llvm-project/pull/193710