Complain about attempts to use 'protected' visibility on targets
like Darwin that don't support it.  We should also complain about
invalid -fvisibility=protected, but that information doesn't seem
to exist at the most appropriate time, so I've left a FIXME behind.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@149186 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Basic/Targets.cpp b/lib/Basic/Targets.cpp
index 5cdb36f..5dc63e7 100644
--- a/lib/Basic/Targets.cpp
+++ b/lib/Basic/Targets.cpp
@@ -218,6 +218,12 @@
     return "__TEXT,__StaticInit,regular,pure_instructions";
   }
 
+  /// Darwin does not support protected visibility.  Darwin's "default"
+  /// is very similar to ELF's "protected";  Darwin requires a "weak"
+  /// attribute on declarations that can be dynamically replaced.
+  virtual bool hasProtectedVisibility() const {
+    return false;
+  }
 };