Strip local symbols when using externalized debug info.

When we build LLVM with externalized debug info, all debugging and
symbolication related data is extracted into dSYM files prior to
stripping. As such, there is no need to preserve local symbols in LLVM
binaries after dSYM creation.

This shrinks libLLVM.dylib from 58MB to 55MB on my system.

llvm-svn: 258566