Reapply "[compiler-rt] Default to Lit's Internal Shell"

This reverts commit 7de242b.

The previous landing contained a logic error where it actually forced
the use of the external shell everywhere, causing some test failures on
Windows. This patch pretty much keeps the existing structure of the
code, just defaulting to the internal shell everywhere instead of only
on Windows.

Author: boomanaiden154

compiler-rt/test/lit.common.cfg.py

@@ -113,16 +113,18 @@ def push_dynamic_library_lookup_path(config, new_path):
         config.environment[dynamic_library_lookup_var] = new_ld_library_path_64
 
 
+# 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.
+#
 # Choose between lit's internal shell pipeline runner and a real shell.  If
 # LIT_USE_INTERNAL_SHELL is in the environment, we use that as an override.
 use_lit_shell = os.environ.get("LIT_USE_INTERNAL_SHELL")
 if use_lit_shell:
     # 0 is external, "" is default, and everything else is internal.
     execute_external = use_lit_shell == "0"
 else:
-    # Otherwise we default to internal on Windows and external elsewhere, as
-    # bash on Windows is usually very slow.
-    execute_external = not sys.platform in ["win32"]
+    # Otherwise we default to internal everywhere.
+    execute_external = False
 
 # Allow expanding substitutions that are based on other substitutions
 config.recursiveExpansionLimit = 10