Switch prelude to a file and refactor lit testing to pass commands directly. (#966)

executable_semantics/testdata/lit.cfg.py

@@ -0,0 +1,39 @@
+__copyright__ = """
+Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+Exceptions. See /LICENSE for license information.
+SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+"""
+
+import lit.formats
+import os
+
+
+# This is a provided variable, ignore the undefined name warning.
+config = config  # noqa: F821
+
+
+def fullpath(relative_path):
+    return os.path.join(os.environ["TEST_SRCDIR"], relative_path)
+
+
+config.name = "lit"
+config.suffixes = [".carbon"]
+config.test_format = lit.formats.ShTest()
+
+config.substitutions.append(
+    ("%{prelude}", fullpath("carbon/executable_semantics/data/prelude.carbon"))
+)
+config.substitutions.append(
+    (
+        "%{executable_semantics}",
+        "%s --prelude=%s"
+        % (
+            fullpath("carbon/executable_semantics/executable_semantics"),
+            fullpath("carbon/executable_semantics/data/prelude.carbon"),
+        ),
+    )
+)
+config.substitutions.append(("%{not}", fullpath("llvm-project/llvm/not")))
+config.substitutions.append(
+    ("%{FileCheck}", fullpath("llvm-project/llvm/FileCheck"))
+)