Previously, an extension decl was always considered exported (externally visible to module clients) as long as it extended an exported type. Extensions need to either contain some externally visible member (e.g. a public method) or implement a conformance to a public protcol, though, to actually be exported. Without this fix, the compiler incorrectly requires internal extensions to types that are always available at runtime to have declared availability which would be a nuisance for library authors.
As part of testing this change, I expanded the attr_inlinable_available.swift test case significantly and that prompted me to scrap the copy of the test specific to macCatalyst as it seemed like needing to keep the two tests in sync was going to be a liability going forward. I replaced the deleted test with a couple of more targeted tests that use `-dump-type-refinement-contexts` to verify the effect of `-target-min-inlining-version min` on the root refinement context. Again a macCatalyst version of the test is required because we don't have bots that are configured to make the macCatalyst runtime the "target" OS.
Resolves rdar://91382040
