[5.5-05142021][Frontend] Do not output swiftinterface when there has been errors

[bnbarham]

Cherry-pick #37523

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When allowing errors with
-experimental-allow-module-with-compiler-errors, do not output the
.swiftinterface when there has been errors. There's no real need to
output them for invalid modules, so this avoids module interface
printing having to have checks for normally-impossible cases.

Resolves rdar://78039608.

[bnbarham]

@swift-ci please test

[bnbarham]

@swift-ci please nominate

Explanation:
Background indexing now has its own build arena, where source is compiled with an option to allow outputting the .swiftmodule regardless of compiler errors. It was also outputting .swiftinterfaces, which can cause crashes due to unexpected (previously impossible) conditions - eg. when attempting to print an extension with an invalid extended nominal type. This change prevents outputting any supplementary outputs other than the ObjectiveC header when there have been errors, since they wouldn't be useful for indexing anyway.

Radar/SR Issue: rdar://78039608

Risk: Low. This change only affects the new index build arena and prevents supplementary output files from being output when there are errors.

Testing: Added a test case to check the supplementary files are no longer output.