[ADT] Drop llvm::Optional clang-specific optmization for trivially copyable types

Summary:
This fixes libLLVM.so ABI mismatches between llvm compiled with clang
and llvm compiled with gcc (PR39427).

Reviewers: bkramer, sylvestre.ledru, mgorny, hans

Reviewed By: bkramer, hans

Subscribers: dexonsmith, kristina, llvm-commits

Differential Revision: https://reviews.llvm.org/D54540

llvm-svn: 346985