Emit benchmark tests into the benchmarks build directory, not lib/
Emit benchmark tests into the benchmarks build directory, not lib/

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@300533 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx