Reduce allocations from calling ITypeSymbol.AllInterfaces (#79817)

* Reduce allocations from calling ITypeSymbol.AllInterfaces

This method accounts for 2.0% of allocations during completion in the CompletionInCohosting Razor speedometer test.

Razor uses this method from a couple disparate locations (mostly around detecting taghelpers and components). I've verified locally that about 2/3 of these allocations can be removed by having the InterfaceInfo cache the public symbol array information too.

Author: ToddGrun

src/Compilers/CSharp/Portable/Symbols/PublicModel/TypeSymbol.cs

@@ -11,6 +11,8 @@ namespace Microsoft.CodeAnalysis.CSharp.Symbols.PublicModel
 {
     internal abstract class TypeSymbol : NamespaceOrTypeSymbol, ISymbol, ITypeSymbol
     {
+        private ImmutableArray<INamedTypeSymbol> _allInterfaces = default;
+
         protected TypeSymbol(CodeAnalysis.NullableAnnotation nullableAnnotation)
         {
             NullableAnnotation = nullableAnnotation;
@@ -108,7 +110,12 @@ ImmutableArray<INamedTypeSymbol> ITypeSymbol.AllInterfaces
         {
             get
             {
-                return UnderlyingTypeSymbol.AllInterfacesNoUseSiteDiagnostics.GetPublicSymbols();
+                if (_allInterfaces.IsDefault)
+                {
+                    ImmutableInterlocked.InterlockedInitialize(ref _allInterfaces, UnderlyingTypeSymbol.AllInterfacesNoUseSiteDiagnostics.GetPublicSymbols());
+                }
+
+                return _allInterfaces;
             }
         }