define __weak to null in rewritten source for 
-fms-extensions as well.

llvm-svn: 93569
diff --git a/clang/lib/Frontend/RewriteObjC.cpp b/clang/lib/Frontend/RewriteObjC.cpp
index 9f777d6..44a770a 100644
--- a/clang/lib/Frontend/RewriteObjC.cpp
+++ b/clang/lib/Frontend/RewriteObjC.cpp
@@ -618,6 +618,7 @@
     Preamble += "#undef __OBJC_RW_DLLIMPORT\n";
     Preamble += "#undef __OBJC_RW_STATICIMPORT\n";
     Preamble += "#define __attribute__(X)\n";
+    Preamble += "#define __weak\n";
   }
   else {
     Preamble += "#define __block\n";