Compiler integration and clean up for QIR generation

[bettinaheim]

This PR

• splits out the QIR data structures that are not specific to Q#,
• cleans up the API of the QIR Generator
• properly integrates the QIR generation pass into the compiler
• adds an additional pass to arrange the target intrinsics in the format that the QIR generator expects them (one specialization per intrinsic callable, tagged with the appropriate attributes)
• enables QIR emission as part of the Sdk (i.e. via project property)
• contains some minor clean-up to the docs generator extension
• ensures that the concrete call graph always contains all specializations for a used callable

[ScottCarda-MS]

Would it be possible to split this PR into several different PRs into a feature branch so that the content can be more easily reviewed? It seems like some of these changes could be reviewed independently.