Add a check in cache.erase() to never attempt it in FROZEN_CACHE mode. (#25259)

If external logic is correct, then this exception should never be
thrown.

(I considered no-opping this call in FROZEN_CACHE mode, but I think that
is not a good idea. Better to fix any callers to cache.erase() in
FROZEN_CACHE mode directly)

Author: juj

tools/cache.py

@@ -83,6 +83,8 @@ def ensure():
 
 def erase():
   ensure_setup()
+  assert not config.FROZEN_CACHE, 'Cache cannot be erased when FROZEN_CACHE is set'
+
   with lock('erase'):
     # Delete everything except the lockfile itself
     utils.delete_contents(cachedir, exclude=[os.path.basename(cachelock_name)])