Improve performance of source data lookup

[nohwnd]

Perf improvement from #1908:

3. We can then replace this code which is reflection loading the assembly and traversing all the contained types and methods with code to do the same visitation using System.Reflection.Metadata.MetadataReader. And we can look up source information for the visited methods in the pdb MetadataReader in 2.

Avoiding reflection assembly load and type resolution in 3 above would also speed up the source information lookup (which we need to perform on every build for non-C# and non-VB projects) and also avoid concerns around loading the test assemblies into the test host process...

AB#1865120

[nohwnd]

Adding to sprint, because it looks like an reasonably simple perf improvement that we could address in some future sprint.