Epic: Memory optimizations

[ladipro]

The following issues are centered around memory and allocations. Most of them are marked with size:1 because they track the investigation which we choose to time-box to one day.

• Optimize CPU and memory allocations of /flp:v=diag and /clp:v=diag #7022
• Investigate leveraging ArrayPool #6710
• Reduce List<ProjectRootElement> allocations in Evaluator.cs #6307
• Replace CopyOnReadEnumerable with immutable collections #6176
• Investigate getting rid of locker object on LazyFormattedBuildEventArgs #6002
• Further improve CreateManifestResourceName memory allocations #5551
• Communication between VS -> MSBuild is pushing 6% (100MB) of allocations onto the LOH #3953
• Consider replacing the evaluator List<T> usage with block lists #3582
• Improve ReuseableStringBuilderFactory #2697
• ExpandItemVectorIntoString interns string which gets on the large object heap #2678
• Hash.Execute() allocates a string which gets to the large object heap. #7086
• ResolveAssemblyReference is producing 70% of node's data building Roslyn.sln #2657
• Lookup/ItemDictionary make up a huge amount of allocations/CPU item #2587
• Use Span-based path manipulation #6977
• Investigate removing CultureName or CultureInfo field from LazyFormattedBuildEventArgs #7015

TODO: Continue grooming the backlog. The last issue above is where we left off.