[codeview] Pass through vftable shape information

The shape is really just the number of methods in the vftable, since we
don't support 16 bit far calls. All calls are near. Encode this number
in the size of the artificial __vtbl_ptr_type DIDerivedType that we
generate. For DWARF, this will be a normal pointer, but for codeview
this will be a wide pointer that gets pattern matched into a
VFTableShape record. Insert this type into the element list of all
dynamic classes when emitting CodeView, so that the backend can emit the
shape even if the vptr lives in a primary base class.

Fixes PR28150

llvm-svn: 280255