These tests are using FileCheck to check the result of diagnostic
formatting in ways that don't match the new formatter. Force the old
formatter or, where possible, generalize so that they match both
formatters.
Implementation-only dependencies may be referenced from internal decls.
When that module is imported as @testable, clients see the internal
decls and may fail accessing them if the transitive implementation-only
dependencies are not loaded.
Let's consider such transtive implementation-only dependencies as
optional for @testable imports. As such, the compiler will attempt to
load them for test targets, and won't fail if the dependency is missing.
We can make these dependencies required for non-public imports, but it
could be project breaking to do so for implementation-only dependencies.
Considering them as optional is a decent compromise.
rdar://79459263
