* Support System.Guid marshalling via VARIANT

VARIANT marshalling in .NET 5+ requires a TLB
for COM records (i.e., ValueType instances). This
means that without a runtime provided TLB, users
must define their own TLB for runtime types or
define their own transfer types.

We address this here by deferring to the NetFX
mscorlib's TLB.

Co-authored-by: Elinor Fung <elfung@microsoft.com>