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

This reverts commit 206a1d2b5b.

The issues that came up with the last landing have been fixed, so trying
this again to see if it sticks this time.

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

@@ -113,6 +113,9 @@ 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")
@@ -120,9 +123,8 @@ 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