[ios] Use a separate module cache directory for each compilation

The swift compiler module cache is brittle with regards to change
in the cached dependent modules and with regards to corruption
(e.g. process interrupted while writing the cache content on disk).

Use a distinct module cache directory for each module compilation
and blow it after the compilation completes (even with failure) to
ensure that each invocation is run in a clean environment.

A better alternative would be to list all the module cache files
as outputs of the compilation, but it is difficult to do without
inspecting the compiled files (and replicating the logic of the
compiler which uses hashes as part of the names of the output).

Bug: 1358073
Change-Id: I53d47fe63b5a3b5d5e8c7bbb8ef17e49d4dd3a42
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3865915


Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Mike Pinkerton <pinkerton@chromium.org>
Commit-Queue: Mike Pinkerton <pinkerton@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1040977}