[cmake] Fix builds with LLVM_ENABLE_PIC=0

Summary:
When this flag is specified, the target llvm-lto is not built, but is still
used as a dependency of the test targets. cmake 2.8 silently ignored this
situation, but with cmake_minimum_required(3.4) it becomes an error. Fix this
by avoiding the inclusion of the target as a dependency.

Reviewers: beanz

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D20882

llvm-svn: 271530