[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 the target as a dependency.

Reviewers: beanz

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D20883

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@271533 91177308-0d34-0410-b5e6-96231b3b80d8