Use the Itanium ABI for thread_local on Darwin.

After some discussion, it was decided to use the Itanium ABI for thread_local on
Darwin OS X platforms. This involved a couple of changes. First, we use
"_tlv_atexit" instead of "__cxa_thread_atexit". Secondly, the global variables
are marked with 'internal' linkage, because we want all access to be calls to
the Itanium-specific entry point, which has normal linkage.
<rdar://problem/13733006>

llvm-svn: 180941

clang/lib/CodeGen/CodeGenModule.cpp

@@ -1916,7 +1916,13 @@ CodeGenModule::GetLLVMLinkageVarDefinition(const VarDecl *D,
            !D->getAttr<WeakImportAttr>()) {
     // Thread local vars aren't considered common linkage.
     return llvm::GlobalVariable::CommonLinkage;
-  }
+  } else if (D->getTLSKind() == VarDecl::TLS_Dynamic &&
+             getTarget().getTriple().isMacOSX())
+    // On Darwin, the backing variable for a C++11 thread_local variable always
+    // has internal linkage; all accesses should just be calls to the
+    // Itanium-specified entry point, which has the normal linkage of the
+    // variable.
+    return llvm::GlobalValue::InternalLinkage;
   return llvm::GlobalVariable::ExternalLinkage;
 }