"Reapply "[Clang] Enable lit internal shell by default"

This reverts commit 32de3b9.

This relands the enablement of the internal shell for clang by default.
This was reverted last time for some Z3 failures that were fixed in
2d550b9. There was also issues around ulimit values persisting
causing test failures in LLVM. Those were addressed in 93dd17a.

Author: boomanaiden154

clang/test/lit.cfg.py

@@ -18,11 +18,22 @@
 # name: The name of this test suite.
 config.name = "Clang"
 
+# TODO: Consolidate the logic for turning on the internal shell by default for all LLVM test suites.
+# See https://github.com/llvm/llvm-project/issues/106636 for more details.
+#
+# We prefer the lit internal shell which provides a better user experience on failures
+# and is faster unless the user explicitly disables it with LIT_USE_INTERNAL_SHELL=0
+# env var.
+use_lit_shell = True
+lit_shell_env = os.environ.get("LIT_USE_INTERNAL_SHELL")
+if lit_shell_env:
+    use_lit_shell = lit.util.pythonize_bool(lit_shell_env)
+
 # testFormat: The test format to use to interpret tests.
 #
 # For now we require '&&' between commands, until they get globally killed and
 # the test runner updated.
-config.test_format = lit.formats.ShTest(not llvm_config.use_lit_shell)
+config.test_format = lit.formats.ShTest(execute_external=not use_lit_shell)
 
 # suffixes: A list of file extensions to treat as test files.
 config.suffixes = [