CodeGeneration should not generate line pragmas from unknown file paths

[NTaylorMullen]

In the case of MVC extensions we generate dynamic _ViewImports.cshtml file equivalents that don't end up having any actual existence on disk. When one of those files has an @using Foo we codegen a line pragma for Foo that does not have a filename. We don't want innaccurate line pragma errors due to lack of file name so we shouldn't render a pragma if a filename is not set on a source location.

[NTaylorMullen]

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