lit.cfg: better check for MSYS

When running the tests under Cygwin using non-Cygwin python,
the platform would be Windows and there would be bash on the path,
so this check for MSYS would not work correctly.

llvm-svn: 187737

clang/test/lit.cfg

@@ -245,7 +245,7 @@ if execute_external:
     config.available_features.add('shell')
 
 # Exclude MSYS due to transforming '/' to 'X:/mingwroot/'.
-if not platform.system() in ['Windows'] or lit.getBashPath() == '':
+if not platform.system() in ['Windows'] or not execute_external:
     config.available_features.add('shell-preserves-root')
 
 # ANSI escape sequences in non-dumb terminal