[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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@342204 91177308-0d34-0410-b5e6-96231b3b80d8