Fix pr18174.

Clang outputs LLVM one top level decl at a time. This combined with the
visibility computation code looking for the newest NamespaceDecl would cause
it to produce different results for nested namespaces.

The two options for producing consistent results are
* Delay codegen of anything inside a namespace until the end of the file.
* Don't look for the newest NamespaceDecl.

This patch implements the second option.
This matches the gcc behavior too.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196712 91177308-0d34-0410-b5e6-96231b3b80d8

lib/AST/Decl.cpp

@@ -976,7 +976,7 @@ getExplicitVisibilityAux(const NamedDecl *ND,
                            kind);
 
   // Use the most recent declaration.
-  if (!IsMostRecent) {
+  if (!IsMostRecent && !isa<NamespaceDecl>(ND)) {
     const NamedDecl *MostRecent = ND->getMostRecentDecl();
     if (MostRecent != ND)
       return getExplicitVisibilityAux(MostRecent, kind, true);

test/CodeGenCXX/visibility.cpp

@@ -1295,3 +1295,17 @@ namespace test68 {
   }
   // Check lines at top of file.
 }
+
+namespace test69 {
+  // PR18174
+  namespace foo {
+    void f();
+  }
+  namespace foo {
+    void f() {};
+  }
+  namespace foo __attribute__((visibility("hidden"))) {
+  }
+  // CHECK-LABEL: define void @_ZN6test693foo1fEv
+  // CHECK-HIDDEN-LABEL: define hidden void @_ZN6test693foo1fEv
+}