Fix escape in Python string in lit.cfg (microsoft#6826)

Change Python regexp ' clang\+\+ ' to ' clang\\+\\+' We're trying to
match fixed strings like ` clang++ `, but `\+` is not a valid Python
escape sequence. Use `\\+` so the regexp machinery sees `\+`

Author: dneto0

tools/clang/test/lit.cfg

@@ -308,7 +308,7 @@ else:
 config.substitutions.append(
     (' clang ', """*** Do not use 'clang' in tests, use '%clang'. ***""") )
 config.substitutions.append(
-    (' clang\+\+ ', """*** Do not use 'clang++' in tests, use '%clangxx'. ***"""))
+    (' clang\\+\\+ ', """*** Do not use 'clang++' in tests, use '%clangxx'. ***"""))
 config.substitutions.append(
     (' clang-cc ',
      """*** Do not use 'clang-cc' in tests, use '%clang_cc1'. ***""") )