Reduce allocations in AbstractTypeMap

Partial fix for dotnet#68996

Both SubstituteTypesWithoutModifiers and SubstituteNamedTypes allocate a temporary array that is used to potentially return an immutable array. Previously, this immutable array was allocated if needed, whereas we can not perform the extra allocation by utilizing ImmutableCollectionsMarshal as the array being wrapped is local to this method.

src/Compilers/CSharp/Portable/Symbols/AbstractTypeMap.cs

@@ -8,6 +8,7 @@
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;
+using System.Runtime.InteropServices;
 using Microsoft.CodeAnalysis.PooledObjects;
 using Roslyn.Utilities;
 
@@ -313,7 +314,7 @@ internal ImmutableArray<TypeSymbol> SubstituteTypesWithoutModifiers(ImmutableArr
                 }
             }
 
-            return result != null ? result.AsImmutableOrNull() : original;
+            return result != null ? ImmutableCollectionsMarshal.AsImmutableArray(result) : original;
         }
 
         internal ImmutableArray<TypeWithAnnotations> SubstituteTypes(ImmutableArray<TypeWithAnnotations> original)
@@ -430,7 +431,7 @@ internal ImmutableArray<NamedTypeSymbol> SubstituteNamedTypes(ImmutableArray<Nam
                 }
             }
 
-            return result != null ? result.AsImmutableOrNull() : original;
+            return result != null ? ImmutableCollectionsMarshal.AsImmutableArray(result) : original;
         }
     }
 }