[Clang] Fix conflict resolution use return instead of break

To avoid causing unexpected failures
clang/test/CodeGenSYCL/native_cpu_mangling.cpp:15:73:
error: cannot mangle this calling convention yet

Author: sys-ce-bb

clang/lib/AST/MicrosoftMangle.cpp

@@ -3092,7 +3092,7 @@ void MicrosoftCXXNameMangler::mangleCallingConvention(CallingConv CC,
         // Currently we only allow this convention in
         // SYCLNativeCPU and raise the usual error otherwise.
         llvm_unreachable("Unsupported CC for mangling");
-      break;
+      return;
   }
 
   DiagnosticsEngine &Diags = Context.getDiags();