[libcxx][dsl] Avoid false-positive cache hits

[arichardson]

Summary

The libc++ dsl gained a cache to improve startup performance in
5390c5a96e9774f0f7d748b5a466c308f916ecd0, but it turns out this cache
can return cache hits even when it shouldn't.
We currently pass c.substitutions as one of the cache keys. However, this
means we are storing a reference to a mutable list as the cache key.
I added some debugging code that modifies config.substitutions between
checks and it turns out that the cache hits even though it shouldn't.
As a workaround for this issue, we now store a deep copy of the cache key
in the cache. I plan to submit a follow-up change that only saves the
relevant substitutions to reduce the amount of copying.

[arichardson]

Abandoning in favour of the committed solution (which also allows using a dict for lookup instead): https://reviews.llvm.org/rGddb2baf9fbff3d574c6c2bd69c2c9569100509a4

libcxx/utils/libcxx/test/dsl.py

 #===----------------------------------------------------------------------===##
 #
 # Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 # See https://llvm.org/LICENSE.txt for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 #
 #===----------------------------------------------------------------------===##
 
+import copy
 import os
 import pipes
 import platform
 import re
 import tempfile
 
 import libcxx.test.format
 import lit
def _memoizeExpensiveOperation(extractCacheKey):
   def decorator(function):
     cache = []
     def f(*args, **kwargs):
       cacheKey = extractCacheKey(*args, **kwargs)
       try:
         result = next(res for (key, res) in cache if key == cacheKey)
       except StopIteration: # This wasn't in the cache
         result = function(*args, **kwargs)
-        cache.append((cacheKey, result))
+        # Note: we have to use copy.deepcopy() here to avoid storing a
+        # mutable list in the cache, since that would cause cache hits
+        # even when the list changed compared to when the value was cached.
+        cache.append((copy.deepcopy(cacheKey), result))
       return result
     return f
   return decorator
 
 def _executeScriptInternal(test, commands):
   """
   Returns (stdout, stderr, exitCode, timeoutInfo)