Tweak bail out from unused member analyzer for struct layout types

Current implementation bailed out in symbol start action, which caused both analysis and reporting to be skipped for unused members in a type with StructLayout attribute. New implementation bails out in symbol end action, so we only skip reporting not analysis.

Fixes #32727

src/EditorFeatures/CSharpTest/RemoveUnusedMembers/RemoveUnusedMembersTests.cs

@@ -2084,6 +2084,26 @@ class C
 }}");
         }
 
+        [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsRemoveUnusedMembers)]
+        [WorkItem(32727, "https://github.com/dotnet/roslyn/issues/32727")]
+        public async Task NestedStructLayoutTypeWithReference()
+        {
+            await TestDiagnosticMissingAsync(
+@"using System.Runtime.InteropServices;
+
+class Program
+{
+    private const int [|MAX_PATH|] = 260;
+
+    [StructLayout(LayoutKind.Sequential)]
+    internal struct ProcessEntry32
+    {
+        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = MAX_PATH)]
+        public string szExeFile;
+    }
+}");
+        }
+
         [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsRemoveUnusedMembers)]
         public async Task FixAllFields_Document()
         {

src/Features/Core/Portable/RemoveUnusedMembers/AbstractRemoveUnusedMembersDiagnosticAnalyzer.cs

@@ -154,13 +154,6 @@ private void RegisterActions(CompilationStartAnalysisContext compilationStartCon
                 Action<ISymbol, ValueUsageInfo> onSymbolUsageFound = OnSymbolUsage;
                 compilationStartContext.RegisterSymbolStartAction(symbolStartContext =>
                 {
-                    if (symbolStartContext.Symbol.GetAttributes().Any(a => a.AttributeClass == _structLayoutAttributeType))
-                    {
-                        // Bail out for types with 'StructLayoutAttribute' as the ordering of the members is critical,
-                        // and removal of unused members might break semantics.
-                        return;
-                    }
-
                     var hasInvalidOrDynamicOperation = false;
                     symbolStartContext.RegisterOperationAction(AnalyzeMemberReferenceOperation, OperationKind.FieldReference, OperationKind.MethodReference, OperationKind.PropertyReference, OperationKind.EventReference);
                     symbolStartContext.RegisterOperationAction(AnalyzeFieldInitializer, OperationKind.FieldInitializer);
@@ -351,6 +344,13 @@ private void OnSymbolEnd(SymbolAnalysisContext symbolEndContext, bool hasInvalid
                     return;
                 }
 
+                if (symbolEndContext.Symbol.GetAttributes().Any(a => a.AttributeClass == _structLayoutAttributeType))
+                {
+                    // Bail out for types with 'StructLayoutAttribute' as the ordering of the members is critical,
+                    // and removal of unused members might break semantics.
+                    return;
+                }
+
                 // Report diagnostics for unused candidate members.
                 var first = true;
                 PooledHashSet<ISymbol> symbolsReferencedInDocComments = null;