Type Visitor bypassing codegen fallback visitors

[kumarak]

Type visitor(CodeGenTypeVisitorWithDataLayoutMixin) bypasses codegen fallback visitor while generating the entries of the data layout.

https://github.com/trailofbits/vast/blob/master/include/vast/Translation/CodeGenTypeVisitor.hpp#L183
https://github.com/trailofbits/vast/blob/master/include/vast/Translation/CodeGenTypeVisitor.hpp#L188

This may cause a clang::type to be mapped to null mlir::Type if it is not supported in the vast type visitor. There is also no way to catch them in the fallback visitor and generate mlir::Type.

The example is DecayedType or AttributedType that is not supported in the vast type visitor and may get mapped to null mlir::Type.