[codeview] Don't emit vshape info for classes without vfptrs

Classes with no virtual methods or whose virtual methods were all
inherited from virtual bases don't have a vfptr at offset zero. We were
crashing attempting to get the layout of that non-existent vftable.

We don't need any vshape info in this case because the debugger can
infer it from the base class information. The current class may not
introduce any virtual methods if we are in this situation.

llvm-svn: 280287