The was never invoked because inaccessibility due to SPI protection level is
always diagnosed before missing imports are diagnosed. The functionality could
therefore not be tested and should be removed.

…ItLoc()`.

For clarity, it should just take a `SourceFile`.

Don't skip checking if a feature is enabled for migration when the feature also
has an associated language version.

The migration to `MemberImportVisibility` can be performed mechanically by
adding missing import declarations, so offer automatic migration for the
feature.

Resolves rdar://151931597.

We would previously unconditionally create the accessor as a throwing
accessor irrespective of the parameter. This seems like an oversight and
results in an incorrect accessor being formed. This was caught while
trying to improve the ClangImporter's handling of aliasing macros.

Also removed concrete SIMDMask(lowHalf:highHalf:) init, as there is no
corresponding generic operation; it was added in error.

…bility

[test] Add a few more known type-checker crashers

There's no reason for these to ever be calls, so they should be
transparent instead of just aEIC. Also adds concrete versions of
comparisons with scalars, and filecheck tests to make sure these
generate 1-2 instruction sequences in release on arm64 (x86_64 is a
little trickier to test due to frame pointers, but if we get the right
codgen on arm64, in practice we do well on x86_64 for these too).

Also makes filecheck patterns for repeating initializers a bit more
robust.