AST: Fix MemberImportVisibility handling of @_exported imports.
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.
