[tests] Reapply r188354, ensure subsitution string is ascii.

 - Now fixed to ensure substitution value isn't of unicode type on Python 2.6-7.

llvm-svn: 188384

clang/test/lit.cfg

@@ -200,7 +200,8 @@ def getClangBuiltinIncludeDir(clang):
     if sys.platform in ['win32'] and execute_external:
         # Don't pass dosish path separator to msys bash.exe.
         dir = dir.replace('\\', '/')
-    return dir
+    # Ensure the result is an ascii string, across Python2.5+ - Python3.
+    return str(dir.decode('ascii'))
 
 config.substitutions.append( ('%clang_cc1', '%s -cc1 -internal-isystem %s'
                               % (config.clang,