[asan] Change the visibility of ___asan_globals_registered to hidden

[kubamracek]

Summary

This flag is used to track global registration in Mach-O and it doesn't need to be exported and visible.

llvm/trunk/lib/Transforms/Instrumentation/AddressSanitizer.cpp

if (UseMachOGlobalsSection) {
     // to look up the loaded image that contains it. Second, we can store in it
     // whether registration has already occurred, to prevent duplicate
     // registration.
     //
     // common linkage ensures that there is only one global per shared library.
     RegisteredFlag = new GlobalVariable(
         M, IntptrTy, false, GlobalVariable::CommonLinkage,
         ConstantInt::get(IntptrTy, 0), kAsanGlobalsRegisteredFlagName);
+    RegisteredFlag->setVisibility(GlobalVariable::HiddenVisibility);
 
     // Update llvm.compiler.used, adding the new liveness globals. This is
     // needed so that during LTO these variables stay alive. The alternative
     // would be to have the linker handling the LTO symbols, but libLTO
     // current API does not expose access to the section for each symbol.
     if (!LivenessGlobals.empty())
       appendToCompilerUsed(M, LivenessGlobals);
   } else if (UseMetadataArray) {