libc++ config testing: `use_system_cxx_lib` can be a bool
libc++ config testing: `use_system_cxx_lib` can be a bool

Fix the libc++ Green Dragon bot.

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@299055 91177308-0d34-0410-b5e6-96231b3b80d8

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