Make GCMemoryInfoData blittable (#58738)

The runtime side makes assumptions about the layout of this class, but since the class is not blittable, the runtime is free to reorder the fields however it sees fit (`StructLayout(LayoutKind.Sequential)` only affects the *managed* layout of blittable classes - it's not documented to do anything to the managed layout of non-blittable classes).

This prevents improvements to managed layout such as #51416.

Co-authored-by: Jan Kotas <jkotas@microsoft.com>

src/coreclr/vm/comutilnative.h

@@ -83,13 +83,7 @@ struct GCGenerationInfo
     UINT64 fragmentationAfter;
 };
 
-#if defined(TARGET_X86) && !defined(TARGET_UNIX)
 #include "pshpack4.h"
-#ifdef _MSC_VER 
-#pragma warning(push)
-#pragma warning(disable:4121) // alignment of a member was sensitive to packing
-#endif
-#endif
 class GCMemoryInfoData : public Object
 {
 public:
@@ -107,6 +101,7 @@ class GCMemoryInfoData : public Object
     UINT32 pauseTimePercent;
     UINT8 isCompaction;
     UINT8 isConcurrent;
+    UINT8 padding[6];
     GCGenerationInfo generationInfo0;
     GCGenerationInfo generationInfo1;
     GCGenerationInfo generationInfo2;
@@ -115,12 +110,7 @@ class GCMemoryInfoData : public Object
     UINT64 pauseDuration0;
     UINT64 pauseDuration1;
 };
-#if defined(TARGET_X86) && !defined(TARGET_UNIX)
-#ifdef _MSC_VER
-#pragma warning(pop)
-#endif
 #include "poppack.h"
-#endif
 
 #ifdef USE_CHECKED_OBJECTREFS
 typedef REF<GCMemoryInfoData> GCMEMORYINFODATA;

src/libraries/System.Private.CoreLib/src/System/GCMemoryInfo.cs

@@ -68,8 +68,8 @@ internal sealed class GCMemoryInfoData
         internal long _index;
         internal int _generation;
         internal int _pauseTimePercentage;
-        internal bool _compacted;
-        internal bool _concurrent;
+        internal byte _compacted;
+        internal byte _concurrent;
 
         private GCGenerationInfo _generationInfo0;
         private GCGenerationInfo _generationInfo1;
@@ -158,12 +158,12 @@ internal GCMemoryInfo(GCMemoryInfoData data)
         /// <summary>
         /// Is this a compacting GC or not.
         /// </summary>
-        public bool Compacted => _data._compacted;
+        public bool Compacted => _data._compacted != 0;
 
         /// <summary>
         /// Is this a concurrent GC (BGC) or not.
         /// </summary>
-        public bool Concurrent => _data._concurrent;
+        public bool Concurrent => _data._concurrent != 0;
 
         /// <summary>
         /// Total committed bytes of the managed heap.