Compiler: Rework DefaultTagHelperDescriptorFactory and RequiredAttributeParser (#11977)

I recommend reviewing this commit-by-commit. In particular, the first
commit (2bea412) is a big one that
could potentially be skipped.

The key changes are in `RequiredAttributeParser` and
`DefaultTagHelperDescriptorFactory` to reduce allocations and clean up
old logic.

----
CI Build:
https://dev.azure.com/dnceng/internal/_build/results?buildId=2736616&view=results
Test Insertion:
https://dev.azure.com/devdiv/DevDiv/_git/VS/pullrequest/646136
Toolset Run:
https://dev.azure.com/dnceng/internal/_build/results?buildId=2736618&view=results

Author: DustinCampbell

src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/CSharp/DefaultTagHelperDescriptorFactory.cs

@@ -24,7 +24,7 @@ public override void Execute(TagHelperDescriptorProviderContext context)
         }
 
         var targetSymbol = context.TargetSymbol;
-        var factory = new DefaultTagHelperDescriptorFactory(compilation, context.IncludeDocumentation, context.ExcludeHidden);
+        var factory = new DefaultTagHelperDescriptorFactory(context.IncludeDocumentation, context.ExcludeHidden);
         var collector = new Collector(compilation, targetSymbol, factory, tagHelperTypeSymbol);
         collector.Collect(context);
     }