Add support for "extended" layouts in the runtimes and add CStruct layout

eng/Versions.props

@@ -122,6 +122,7 @@
     <SystemValueTupleVersion>4.6.1</SystemValueTupleVersion>
     <!-- Libraries dependencies -->
     <MicrosoftBclAsyncInterfacesVersion>6.0.0</MicrosoftBclAsyncInterfacesVersion>
+    <MicrosoftBclMemoryVersion>9.0.5</MicrosoftBclMemoryVersion>
     <MicrosoftWin32RegistryVersion>5.0.0</MicrosoftWin32RegistryVersion>
     <StyleCopAnalyzersVersion>1.2.0-beta.556</StyleCopAnalyzersVersion>
     <SystemComponentModelAnnotationsVersion>5.0.0</SystemComponentModelAnnotationsVersion>

src/coreclr/System.Private.CoreLib/src/System/Reflection/Emit/RuntimeTypeBuilder.cs

@@ -519,12 +519,6 @@ private void VerifyTypeAttributes(TypeAttributes attr)
                 }
             }
 
-            // Verify that the layout mask is valid.
-            if (((attr & TypeAttributes.LayoutMask) != TypeAttributes.AutoLayout) && ((attr & TypeAttributes.LayoutMask) != TypeAttributes.SequentialLayout) && ((attr & TypeAttributes.LayoutMask) != TypeAttributes.ExplicitLayout))
-            {
-                throw new ArgumentException(SR.Argument_BadTypeAttrInvalidLayout);
-            }
-
             // Check if the user attempted to set any reserved bits.
             if ((attr & TypeAttributes.ReservedMask) != 0)
             {

src/coreclr/System.Private.CoreLib/src/System/Reflection/RuntimeCustomAttributeData.cs

@@ -2272,6 +2272,7 @@ internal static bool IsDefined(RuntimeFieldInfo field, RuntimeType? caType)
                 case TypeAttributes.ExplicitLayout: layoutKind = LayoutKind.Explicit; break;
                 case TypeAttributes.AutoLayout: layoutKind = LayoutKind.Auto; break;
                 case TypeAttributes.SequentialLayout: layoutKind = LayoutKind.Sequential; break;
+                case TypeAttributes.ExtendedLayout: layoutKind = LayoutKind.Extended; break;
                 default: Debug.Fail("Unreachable code"); break;
             }
 

src/coreclr/ilasm/asmparse.y

@@ -76,7 +76,7 @@
 
 %token VALUE_ VALUETYPE_ NATIVE_ INSTANCE_ SPECIALNAME_ FORWARDER_
 %token STATIC_ PUBLIC_ PRIVATE_ FAMILY_ FINAL_ SYNCHRONIZED_ INTERFACE_ SEALED_ NESTED_
-%token ABSTRACT_ AUTO_ SEQUENTIAL_ EXPLICIT_ ANSI_ UNICODE_ AUTOCHAR_ IMPORT_ ENUM_
+%token ABSTRACT_ AUTO_ SEQUENTIAL_ EXPLICIT_ EXTENDED_ ANSI_ UNICODE_ AUTOCHAR_ IMPORT_ ENUM_
 %token VIRTUAL_ NOINLINING_ AGGRESSIVEINLINING_ NOOPTIMIZATION_ AGGRESSIVEOPTIMIZATION_ UNMANAGEDEXP_ BEFOREFIELDINIT_ ASYNC_
 %token STRICT_ RETARGETABLE_ WINDOWSRUNTIME_ NOPLATFORM_
 %token METHOD_ FIELD_ PINNED_ MODREQ_ MODOPT_ SERIALIZABLE_ PROPERTY_ TYPE_
@@ -260,6 +260,7 @@ id                      : ID                                  { $$ = $1; }
                         | NOOPTIMIZATION_                     { $$ = newString("nooptimization"); }
                         | AGGRESSIVEOPTIMIZATION_             { $$ = newString("aggressiveoptimization"); }
                         | ASYNC_                              { $$ = newString("async"); }
+                        | EXTENDED_                           { $$ = newString("extended"); }
                         | SQSTRING                            { $$ = $1; }
                         ;
 
@@ -444,6 +445,7 @@ classAttr               : /* EMPTY */                       { $$ = (CorRegTypeAt
                         | classAttr AUTO_                   { $$ = (CorRegTypeAttr) (($1 & ~tdLayoutMask) | tdAutoLayout); }
                         | classAttr SEQUENTIAL_             { $$ = (CorRegTypeAttr) (($1 & ~tdLayoutMask) | tdSequentialLayout); }
                         | classAttr EXPLICIT_               { $$ = (CorRegTypeAttr) (($1 & ~tdLayoutMask) | tdExplicitLayout); }
+                        | classAttr EXTENDED_               { $$ = (CorRegTypeAttr) (($1 & ~tdLayoutMask) | tdExtendedLayout); }
                         | classAttr ANSI_                   { $$ = (CorRegTypeAttr) (($1 & ~tdStringFormatMask) | tdAnsiClass); }
                         | classAttr UNICODE_                { $$ = (CorRegTypeAttr) (($1 & ~tdStringFormatMask) | tdUnicodeClass); }
                         | classAttr AUTOCHAR_               { $$ = (CorRegTypeAttr) (($1 & ~tdStringFormatMask) | tdAutoClass); }