[Darwin][NFC] Refactor throttling of 64bit sanitizer tests on Darwin

Underlying condition for throttling is "has large mmap'd regions" (i.e.,
shadow memory) and not sanitizers in general (e.g., UBSan does not need
to be throttled).

Rename parallelism group `darwin-64bit-sanitizer` to `shadow-memory` and
apply it unconditionally to all tests which require it. We can then have
all the Darwin throttling logic in one place in the commen lit config.

Throttle sanitizer_common unit tests. Configuration was previously
missing from sanitizer_common/Unit/lit.site.cfg.

Reviewed by: kubamracek

Differential Revision: https://reviews.llvm.org/D58677

llvm-svn: 355018