[lld] Make lld tests use lit internal shell by default

This patch updates the lld lit test config to use the internal shell by
default. This has some performance advantages (~10-15%) and also
produces nicer failure output. This should have no impact on test
coverage now that all tests previously requiring a shell have been
ported over to work with the internal shell.

Pull Request: llvm#156538

Author: boomanaiden154

lld/test/lit.cfg.py

@@ -16,10 +16,21 @@
 # name: The name of this test suite.
 config.name = "lld"
 
+# 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 = [".ll", ".s", ".test", ".yaml", ".objtxt"]