[asan] Restore ODR-violation detection on vtables

Summary:
unnamed_addr is still useful for detecting of ODR violations on vtables

Still unnamed_addr with lld and --icf=safe or --icf=all can trigger false
reports which can be avoided with --icf=none or by using private aliases
with -fsanitize-address-use-odr-indicator

Reviewers: eugenis

Reviewed By: eugenis

Subscribers: kubamracek, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D55799

llvm-svn: 349555