[test] Use lit's shell test runner on Windows

Summary:
I did a local comparison between using bash and using lit's runner, and
more of the suite passes with lit than passes with bash.  Most of the
bash failures have to do with /dev/null, which is nonsensical on
Windows, but the lit runner handles it.

The lit shell runner is also much faster than bash, so I would expect
most Windows devs would want it by default.

The behavior can be overridden on any OS by setting
LIT_USE_INTERNAL_SHELL to 0 or 1 in the environment.

Reviewers: chapuni, ddunbar

CC: llvm-commits, timurrrr

Differential Revision: http://llvm-reviews.chandlerc.com/D559

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179173 91177308-0d34-0410-b5e6-96231b3b80d8

test/lit.cfg

@@ -22,9 +22,18 @@ if sys.platform in ['win32']:
                                      config.environment['PATH']))
         config.environment['PATH'] = path
 
+# 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'])
+
 # testFormat: The test format to use to interpret tests.
-execute_external = (not sys.platform in ['win32']
-                    or lit.getBashPath() not in [None, ""])
 config.test_format = lit.formats.ShTest(execute_external)
 
 # To ignore test output on stderr so it doesn't trigger failures uncomment this:
@@ -240,7 +249,7 @@ for pattern in [r"\bbugpoint\b(?!-)",   r"(?<!/|-)\bclang\b(?!-)",
 ### Features
 
 # Shell execution
-if sys.platform not in ['win32'] or lit.getBashPath() != '':
+if execute_external:
     config.available_features.add('shell')
 
 # Loadable module