Better diagnostic when building with unsupported language subset

[rauhul]

Description

When building for a target like visionOS or avr which may only have support for a single language subset (like "full swift" or "embedded swift" respectively) we should emit better diagnostics.

For example if you try to build with an unsupported subset today you will see an error like "Failed to load Swift.swiftmodule". Instead we should look for other variants for the same triple and instead a emit an error like: "Failed to find Swift.swiftmodule for full Swift, but found Swift.swiftmodule for Embedded Swift; did you mean to pass '-enable-experimental-feature Embedded'"