Return value of ParseRelativeBoundsForDecl not being used

[secure-sw-dev-bot]

This issue was copied from checkedc/checkedc-clang#282

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I noticed that the return value of ParseRelativeBoundsForDecl seems to never be used. At the call in ParseBoundsExpressionOrInteropType, I think we should check the return value and make sure that the function returns an invalid bounds expression if there was an error. This will prevent spurious downstream errors later on.

I don't think we need to do anything at the call in ParseStructDeclaration. It would be helpful to add a comment there explaining what is going on. We expect this call to always generate an error message. Because of that, we don't need to check the return value.