Allow generating xml and csharp in a single invocation

[minichma]

The ClangSharpPInvokeGenerator tool allows specifying the output format (i.e. csharp or xml) via the -om option. In some cases multiple formats are needed, which requires multiple invocations of the tool. It would be beneficial if we could generate multiple formats in a single invocation. This ticket is a feature request to allow passing the -om option multiple times, specifying multiple output formats or alternatively allow passing a comma separated list to -om.

I.e.

ClangSharpPInvokeGenerator @generate.rsp -om csharp -om xml

or

ClangSharpPInvokeGenerator @generate.rsp -om csharp,xml