Explicitly handle char marshalling in OleVariant::GetNativeMethodTableForVarType.

[jkoritzinsky]

The default handling for char marshalling in OleVariant::GetNativeMethodTableForVarType() doesn't correctly handle char[]s marshaled as ByValArray. The MethodTable* calculates for char elements doesn't correctly account for CharSet.

In master, this code-path is only executed as part of SystemV classification of by-val arrays, so it requires a contrived struct to encounter, such as the one below:

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
struct S
{
    [MarshalAs(UnmanagedType.ByValArray, SizeConst=6)]
    public char[] arr;

	public float f;
}

Currently, this classifies the first eight bytes as INTEGER and the second eight bytes as SSE. However, both eight bytes should be classified as INTEGER since the char array extends into the second eight byte.

I discovered this bug as part of my work on unifying the struct marshaling with the parameter and return value marshaling where I am using the OleVariant::GetNativeMethodTableForVarType() in more locations.

[AaronRobinsonMSFT]

Let's add a test for this case.