This patch makes it an error to have a mismatch between the enabled
sanitizers in a CU, and in any module being imported into the CU. Only
mismatches between non-modular sanitizers are treated as errors.
This patch also includes non-modular sanitizers in module hashes, in
order to ensure module rebuilds occur when -fsanitize=X is toggled on
and off for non-modular sanitizers, and to cut down on module rebuilds
when the option is toggled for modular sanitizers.
This fixes a longstanding issue with implicit modules and sanitizers,
which Duncan originally diagnosed.
When building with implicit modules it's possible to hit a scenario
where modules are built without -fsanitize=address, and are subsequently
imported into CUs with -fsanitize=address enabled. This causes strange
failures at runtime. The case Duncan found affects libcxx, since its
vector implementation behaves differently when ASan is enabled.
Implicit module builds should "just work" when -fsanitize=X is toggled
on and off across multiple compiler invocations, which is what this