[libc++] Persistently cache memoized operations during Lit configuration

[libc++] Persistently cache memoized operations during Lit configuration

When invoking Lit repeatedly, we perform all the configuration checks
over and over again, which takes a lot of time. This patch allows caching
the result of configuration checks persistently across Lit invocations to
speed this up.

In theory, this should still be functionally correct since the cache
key should contain everything that determines the output of the
configuration check. However, in cases where e.g. the compiler has
changed but is at the same path as previously, the Lit configuration
checks will be cached even though technically the cache should have
been invalidated.

Differential Revision: https://reviews.llvm.org/D117361