[Support] Treat null bytes as separator in windows command line strings

When reading directives from a .drectve section, the directives are
tokenized as a normal windows command line. However in these cases,
link.exe allows the directives to be separated by null bytes, not only by
spaces.

A test case for this change will be added in the lld repo.

Differential Revision: https://reviews.llvm.org/D52014

llvm-svn: 342204