Make StackTrace Symbol Caching Aware of Collectible Types (Phase 2) (dotnet/corefx...

Make StackTrace Symbol Caching Aware of Collectible Types (Phase 2) (dotnet/corefx#35827)

* Update GetSourceLineInfo to consume an assembly:
* Use Assembly as cache key for MetadataReader
* Dynamic and regular assemblies now have the same cache key
* Prevents stale cache entries in the case where a PEFile is unloaded
and replaced by a new one in the same location

dotnet/coreclrdotnet/corefx#20179

* Remove redundant overload of GetSourceLineInfo
* update comments to reflect new behavior

* Change TryGetReader to use ConditionalWeakTable's atomic Add with callback method.
* cleaning up var names based on feedback

Commit migrated from https://github.com/dotnet/corefx/commit/6f00e97f85650768f87743df769206bfc4119fbc