Description of Change

[XamlC] Cache Resolve and ImportReference

ImportReference with System.Reflection based argument is notoriously
slow on .NET. So we cache the results for those.

We do not cache the results for TypeReference, MethodReference or
FieldReference calls, as those are already fast (passthrough if the
reference was already imported), and they aren't valid as dictionary
keys (no concept of equatability).

While we're at it, we shave another few ms from Resolve(TypeReference)
calls as well.

As, on good days, it shaves up to 40% of XamlC time, we can say that it fixes #1848

Bugs Fixed

• fixes Improve XAMLC Compilation performance #1848

PR Checklist

• Has tests (if omitted, state reason in description)
• Rebased on top of master at time of PR
• Changes adhere to coding standard
• Consolidate commits as makes sense