Implement the "linker_private_weak" linkage type. This will be used for
Objective-C metadata types which should be marked as "weak", but which the
linker will remove upon final linkage. However, this linkage isn't specific to
Objective-C.

For example, the "objc_msgSend_fixup_alloc" symbol is defined like this:

      .globl l_objc_msgSend_fixup_alloc
      .weak_definition l_objc_msgSend_fixup_alloc
      .section __DATA, __objc_msgrefs, coalesced
      .align 3
l_objc_msgSend_fixup_alloc:
       .quad   _objc_msgSend_fixup
       .quad   L_OBJC_METH_VAR_NAME_1

This is different from the "linker_private" linkage type, because it can't have
the metadata defined with ".weak_definition".

Currently only supported on Darwin platforms.

llvm-svn: 107433
diff --git a/llvm/test/Feature/linker_private_linkages.ll b/llvm/test/Feature/linker_private_linkages.ll
new file mode 100644
index 0000000..19bcbb4
--- /dev/null
+++ b/llvm/test/Feature/linker_private_linkages.ll
@@ -0,0 +1,6 @@
+; RUN: llvm-as < %s | llvm-dis > %t1.ll
+; RUN: llvm-as %t1.ll -o - | llvm-dis > %t2.ll
+; RUN: diff %t1.ll %t2.ll
+
+@foo = linker_private hidden global i32 0
+@bar = linker_private_weak hidden global i32 0