In existing Swift, an `@_exported import` in any source file makes the
declarations from the imported module visible in all source files. It's unclear
whether this is an explicit decision or is simply and unintended consequence of
effectively adding an implicit import to each source file for the module being
compiled.
Although it's not clear whether this behavior is desirable, the behavior of
member lookup when the MemberImportVisibility feature is enabled should align
with it in order to avoid causing unnecessary churn in required imports.
Resolves rdar://132525152.
Control enforcement of member import visibility requirements via a new option,
instead of piggy-backing on the existing IgnoreAccessControl option. Adopt the
option when doing fallback lookups for unviable members so that the compiler
can diagnose the reason that a member is inaccessible more reliably.
Previously, with MemberImportVisibility enabled decls with the package access
level could be mis-diagnosed as inaccessible due to their access level when
really they were inaccessible due to a missing import.
Resolves rdar://131501862.
