Rename -discard-value-names into -lto-discard-value-names in libLLVMLTO

This is avoiding a naming conflict with opt and llc.
While opt and llc don't link to LTO usually, users that are building a
monolithic libLLVM.dylib and linking the tools to it would have a
runtime error because of the duplicate cl::opt registration.

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 263127