Reapply "[llvm] Use lit internal shell by default"

This reverts commit 5125f47.

This was reverted because it broke some debuginfod tests. Those have been fixed
now in #158141. Relanding now and hoping it sticks this time.

Author: boomanaiden154

llvm/test/lit.cfg.py

@@ -17,6 +17,17 @@
 # name: The name of this test suite.
 config.name = "LLVM"
 
+# 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.
 extra_substitutions = extra_substitutions = (
     [
@@ -26,9 +37,7 @@
     if config.enable_profcheck
     else []
 )
-config.test_format = lit.formats.ShTest(
-    not llvm_config.use_lit_shell, extra_substitutions
-)
+config.test_format = lit.formats.ShTest(not use_lit_shell, extra_substitutions)
 
 # suffixes: A list of file extensions to treat as test files. This is overriden
 # by individual lit.local.cfg files in the test subdirectories.