Port r198088 (set NO_DEAD_STRIP for clang) from make to cmake.

Also stop setting passing -dead_strip explicitly for libclang and instead
rely on this now happening by default. (And make it happen by default for
add_clang_library, which doesn't use the library cmake functions from llvm.)


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198200 91177308-0d34-0410-b5e6-96231b3b80d8