Crossgen2 composite framework build emits OptimizedInboxTextEncoder layout mismatched with the CoreCLR runtime #64419

Architecture: 64-bit
OS: all
Example run: https://dev.azure.com/dnceng/public/_build/results?buildId=1574573&view=results

Diagnostic information:

      Assert failure(PID 1408 [0x00000580], Thread: 2176 [0x0880]): Verify_FieldOffset 'System.Text.Encodings.Web.OptimizedInboxTextEncoder._allowedAsciiCodePoints' Field offset 16!=24(actual) || baseOffset 8!=8(actual)
      
      CORECLR! LoadDynamicInfoEntry + 0x11C1 (0x00007ffb`0ec98e71)
      CORECLR! Module::FixupNativeEntry + 0x280 (0x00007ffb`0eb7d190)
      CORECLR! Module::FixupDelayListAux + 0x6AD (0x00007ffb`0ee0d88d)
      CORECLR! ReadyToRunInfo::GetEntryPoint + 0x64A (0x00007ffb`0ee0ff4a)
      CORECLR! MethodDesc::GetPrecompiledR2RCode + 0xDA (0x00007ffb`0ed5eb1a)
      CORECLR! MethodDesc::GetPrecompiledCode + 0xD1 (0x00007ffb`0ed5e711)
      CORECLR! MethodDesc::PrepareILBasedCode + 0x3B0 (0x00007ffb`0ed62030)
      CORECLR! MethodDesc::PrepareCode + 0xE8 (0x00007ffb`0ed61c28)
      CORECLR! CodeVersionManager::PublishVersionableCodeIfNecessary + 0x3F3 (0x00007ffb`0ebe5433)
      CORECLR! MethodDesc::DoPrestub + 0xC0E (0x00007ffb`0ed5aafe)
          File: D:\a\_work\1\s\src\coreclr\vm\jitinterface.cpp Line: 13642
          Image: C:\h\w\AF87095D\p\corerun.exe

At the first glance it seems to me that this is caused by mismatching classification of the type by Crossgen2 and the runtime. Crossgen2 query for

doesn't hit the sequential path because the type does contain GC pointers, however the check in

does classify the type as Blittable and enters the sequential layout path.

/cc @dotnet/crossgen-contrib, @jkoritzinsky