lit: Use sys.executable for executing builtin commands

Summary:
The python executable may not exist on all systems so use sys.executable
instead.

Reviewers: ddunbar, stella.stamenova

Subscribers: delcypher, llvm-commits

Differential Revision: https://reviews.llvm.org/D51511

llvm-svn: 341244

utils/lit/lit/TestRunner.py

@@ -879,7 +879,7 @@ def _executeShCmd(cmd, shenv, results, timeoutHelper):
         # Expand all glob expressions
         args = expand_glob_expressions(args, cmd_shenv.cwd)
         if is_builtin_cmd:
-            args.insert(0, "python")
+            args.insert(0, sys.executable)
             args[1] = os.path.join(builtin_commands_dir ,args[1] + ".py")
 
         # On Windows, do our own command line quoting for better compatibility