Fix explicit layout validator memory alloc problems

[davidwrighton]

The ExplicitLayoutValidator currently used an array of bytes one to one with the size of the layout of the class. However, we have some test cases for outrageously large explicit layout structures and classes which cause this approach to OOM in crossgen2.

The fix is to move to a data structure defined in terms of intervals. This is probably a bit slower, but it requires vastly less memory for pathological situations.

Fixes #55164
Fixes #53559

[davidwrighton]

@dotnet/crossgen-contrib

[MichalStrehovsky]

TIL.

I suggested using a byte array when this was introduced because CoreCLR uses that and it was an easy to follow algorithm. Didn't think we allow such big classes.

[StructLayout(LayoutKind.Explicit, Size = Int32.MaxValue)]
struct Boo { }

[StructLayout(LayoutKind.Explicit)]
class Program
{
    [FieldOffset(0)]
    Boo b;

    static void Main() { }
}

This crashes 5.0 CoreCLR on x86 big time (somewhere in native-land that I didn't debug). Fun.

[trylek]

LGTM, thanks for making this more robust!