Replace the `here` part of the generic exportability diagnostic for
variables with: `in a property declaration marked public or in a
'@frozen' or '@usableFromInline' context`.
The full diagnostic now looks like:
```
error: cannot use struct 'ImportedType' in a property declaration marked
public or in a '@frozen' or '@usableFromInline' context;
'HiddenDependency' has been imported as implementation-only
```
This should be improved further to support implicitly exported memory
layouts in non-library-evolution and embedded.
These are tests that fail in the next commit without this flag. This
does not add -verify-ignore-unrelated to all tests with -verify, only
the ones that would fail without it. This is NFC since this flag is
currently a no-op.
Intro the lit substitution
`%target-swift-emit-module-interfaces(Main.swiftinterface,
Main.private.swiftinterface` to generate both the public and private
swiftinterface. And use it in spi-only-import-exportability.swift.
To verify the generated interfaces I would recommend to keep two calls
to `%target-swift-typecheck-module-from-interface` as each is a
different invocation likely to fail.
