[Serialization] Add warning when .swiftsourceinfo is malformed

[bnbarham]

Cherry-pick #37196:

Output a warning rather than ignoring a malformed .swiftsourceinfo
completely.

Resolves rdar://77350048

[bnbarham]

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - e9728bd

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - e9728bd

[bnbarham]

The change to remove ignoring .swiftsourceinfo by default wasn't in 5.5 yet, it is now.

@swift-ci please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - e9728bd

[bnbarham]

Unrelated failure, looks like it should be fixed now

@swift-ci please test macOS platform

[bnbarham]

Looks like the Windows failure is fixed now too.

@swift-ci please test Windows platform

[bnbarham]

@swift-ci please nominate

--- CCC ---

Explanation: Outputs a warning when reading a malformed .swiftsourceinfo, rather than completely ignoring it. The most common case would be an old version. Even this would be unusual as the .swiftmodule would normally be out-of-date as well, causing both to be rebuilt. In case this does happen, however, this warning will help with diagnosing stale data in SourceKit responses.

Radar/SR Issue: rdar://77350048

Risk: Very minimal. At most there's a risk of a spuriously outputting the new diagnostic warning.

Testing: Added a test case for loading invalid .swiftsourceinfo files.

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - e9728bd

[bnbarham]

Still unrelated

@swift-ci please test macOS platform