[WPD] Avoid noalias assumptions in unique return value optimization

Summary:
Changes the type of the @__typeid_.*_unique_member imports we generate
for unique return value optimization from i8 to [0 x i8]. This
prevents assuming that these imports do not alias, such as when
two unique return values occur in the same vtable.

Fixes PR45393.

Reviewers: tejohnson, pcc

Reviewed By: pcc

Subscribers: aganea, hiraditya, rnk, george.burgess.iv, dblaikie, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77421