[ORC] Add BitReader/BitWriter to target_link_libraries

Summary:
CompileOnDemandLayer.cpp uses function in these libraries, and builds
with `-DSHARED_LIB=ON` fail without this.

Reviewers: lhames

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D48995

llvm-svn: 336389