[lld] Move Threads to Common

Summary:
This will allow using the functionality from other linkers. It is also
a prerequisite for sharing the error logging code.

Reviewers: ruiu

Reviewed By: ruiu

Subscribers: emaste, mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D38822

llvm-svn: 315725
diff --git a/lld/ELF/MapFile.cpp b/lld/ELF/MapFile.cpp
index 968dc611..ad80b06 100644
--- a/lld/ELF/MapFile.cpp
+++ b/lld/ELF/MapFile.cpp
@@ -26,7 +26,8 @@
 #include "Strings.h"
 #include "SymbolTable.h"
 #include "SyntheticSections.h"
-#include "Threads.h"
+
+#include "lld/Common/Threads.h"
 
 #include "llvm/Support/raw_ostream.h"