Remove usage of LLVM_PREFIX.

Summary: LLVM_PREFIX could be undefined if CMAKE_INSTALL_PREFIX were set to empty.

Reviewers: kparzysz, bkramer, chandlerc

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D17784

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@263766 91177308-0d34-0410-b5e6-96231b3b80d8