[RFC] Enhance synchscope representation (clang)

[kzhuravl]

Summary

As described in this proposal: https://groups.google.com/forum/#!topic/llvm-dev/GtWfCc5j-4U

[kzhuravl]

Rebase

[kzhuravl]

Remove metadata generation

[kzhuravl]

This patch is not required at this point since we left SynchronizationScope enum intact and did not rename existing members

lib/CodeGen/CGBuiltin.cpp

   case Builtin::BI__atomic_signal_fence:
   case Builtin::BI__c11_atomic_thread_fence:
   case Builtin::BI__c11_atomic_signal_fence: {
-    llvm::SynchronizationScope Scope;
+    unsigned Scope;
     if (BuiltinID == Builtin::BI__atomic_signal_fence ||
-        BuiltinID == Builtin::BI__c11_atomic_signal_fence)
-      Scope = llvm::SingleThread;
-    else
-      Scope = llvm::CrossThread;
+        BuiltinID == Builtin::BI__c11_atomic_signal_fence) {
+      Scope = llvm::SynchScope::SingleThread;
+    } else {
+      Scope = llvm::SynchScope::System;
+    }
     Value *Order = EmitScalarExpr(E->getArg(0));
     if (isa<llvm::ConstantInt>(Order)) {
       int ord = cast<llvm::ConstantInt>(Order)->getZExtValue();