Empty files should not be analyzed

[sharwell]

Currently some files are being analyzed when they are completely empty (an example appears in #1039).

If a file consists of nothing but whitespace and/or newlines, no warnings should be reported.

[pdelvo]

grabbed

[sharwell]

@pdelvo This is as good a time as any to move this analysis up into DiagnosticVerifier.

[pdelvo]

I'm going to add a few tests for auto generated files

[sharwell]

I'm thinking if VerifyCSharpDiagnosticAsync is called and the expected results contain one or more diagnostic starting with SA or SX, we could automatically run a second test on the same input with /// <auto-generated> added to the beginning and ensure that the result:

1. Does not include anything from the expected results which starts with SA or SX.
2. Does include everything else from the expected results, but with the locations incremented by one line.

This would allow us to automatically check every diagnostic for compliance with the generated code exclusion mechanism without actually writing full tests for each one.