LTOCodeGenerator: add linkonce(_odr) to "llvm.compiler.used" when present in "MustPreserve" set

If the linker requested to preserve a linkonce function, we should
honor this even if we drop all uses.
We explicitely avoid turning them into weak_odr (unlike the first
version of this patch in r267644), because the codegen can be
different on Darwin: because of `llvm::canBeOmittedFromSymbolTable()`
we may emit the symbol as weak_def_can_be_hidden instead of
weak_definition.

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 268607