[PassManager] add ReaderWriter{Native,YAML} to the Driver.

Enable this for the following flavors

a) core
b) gnu
c) darwin

Its disabled for the flavor PECOFF. Convenient markers are added with FIXME
comments in the Driver that would be removed and code removed from each flavor.

llvm-svn: 193585

lld/lib/Core/LinkingContext.cpp

@@ -108,6 +108,6 @@ ErrorOr<File &> LinkingContext::nextFile() {
   }
 }
 
-void LinkingContext::addPasses(PassManager &pm) const {}
+void LinkingContext::addPasses(PassManager &pm) {}
 
 } // end namespace lld