LowerTypeTests: Give imported symbols a type with size 0 so that they are not assumed not to alias.

It is possible for both a base and a derived class to be satisfied
with a unique vtable. If a program contains casts of the same pointer
to both of those types, the CFI checks will be lowered to this
(with ThinLTO):

if (p != &__typeid_base_global_addr)
  trap();
if (p != &__typeid_derived_global_addr)
  trap();

The optimizer may then use the first condition combined
with the assumption that __typeid_base_global_addr and
__typeid_derived_global_addr may not alias to optimize away the second
comparison, resulting in an unconditional trap.

This patch fixes the bug by giving imported globals the type [0 x i8]*,
which prevents the optimizer from assuming that they do not alias.

Differential Revision: https://reviews.llvm.org/D38873

llvm-svn: 315753