CodeGen: __uuidof should work even with an incomplete _GUID type

Summary:
We would crash in CodeGen::CodeGenModule::EmitUuidofInitializer
because our attempt to enter CodeGen::CodeGenModule::EmitConstantValue
will be foiled: the type of the constant value is incomplete.

Instead, create an unnamed type with the proper layout on all platforms.
Punt the problem of wrongly defined struct _GUID types to the user.
(It's impossible because the TU may never get to see the type and thus
we can't verify that it is suitable.)

This fixes PR16856.

Reviewers: rsmith, rnk, thakis

Reviewed By: rnk

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1375