Fix five of the shared library build targets

Before this patch there was a cyclic dependency between lldCore and
lldReaderWriter.  Only lldConfig could be built as a shared library.

* Moved Reader and Writer base classes into lldCore.
* The following shared libraries can now be built:
     lldCore
     lldYAML
     lldNative
     lldPasses
     lldReaderWriter

Differential Revision: http://reviews.llvm.org/D7105

From: Greg Fitzgerald <garious@gmail.com>
llvm-svn: 226732
diff --git a/lld/lib/ReaderWriter/MachO/MachOLinkingContext.cpp b/lld/lib/ReaderWriter/MachO/MachOLinkingContext.cpp
index 24e17d0..9fe1610 100644
--- a/lld/lib/ReaderWriter/MachO/MachOLinkingContext.cpp
+++ b/lld/lib/ReaderWriter/MachO/MachOLinkingContext.cpp
@@ -14,11 +14,11 @@
 #include "MachOPasses.h"
 #include "lld/Core/ArchiveLibraryFile.h"
 #include "lld/Core/PassManager.h"
+#include "lld/Core/Reader.h"
+#include "lld/Core/Writer.h"
 #include "lld/Driver/Driver.h"
 #include "lld/Passes/LayoutPass.h"
 #include "lld/Passes/RoundTripYAMLPass.h"
-#include "lld/ReaderWriter/Reader.h"
-#include "lld/ReaderWriter/Writer.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/Triple.h"
 #include "llvm/Config/config.h"