Use ref assemblies in more locations

[rainersigwald]

Need to confirm these changes with owners of:

• WinMDExp
• LC

Continuation of #2167 for the post-15.3 timeframe.

[rainersigwald]

@yizhang82 Can you confirm that WinMDExp only looks at the public interface of managed assemblies passed to it with /reference:? If so, this change will mean that the ExportWindowsMDFile can be skipped more often in incremental builds, because it will only run when the public interface of a reference changes, rather than when any change is made to the referenced assembly.

[yizhang82]

@rainersigwald Yes, we only care about public contracts in the reference assemblies.

[AndyGerlicher]

Split into two commits and merge.