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

This reverts commit ee5bc57.

This was causing test failures on Darwin due to printf there not supporting hex
escapes. That has since been fixed in a separate patch.

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.