Remove usage of LLVM_PREFIX.
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: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@263766 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang