[PECOFF] Allow multiple directives in one module-definition file.

I'm a bit surprised that I have not implemented this yet. This is
definitely needed to handle real-world module definition files.
This patch contains a unit test for r207294.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@207297 91177308-0d34-0410-b5e6-96231b3b80d8