Implement redeclaration merging for namespaces defined in distinct
modules. Teach name lookup into namespaces to search in each of the
merged DeclContexts as well as the (now-primary) DeclContext. This
supports the common case where two different modules put something
into the same namespace.

llvm-svn: 147778
diff --git a/clang/test/Modules/Inputs/namespaces-left.h b/clang/test/Modules/Inputs/namespaces-left.h
index 85e6d7d..6835cda 100644
--- a/clang/test/Modules/Inputs/namespaces-left.h
+++ b/clang/test/Modules/Inputs/namespaces-left.h
@@ -9,3 +9,19 @@
 namespace N2 { 
   float& f(float);
 }
+
+
+
+
+
+namespace N5 {
+  int &f(int);
+}
+
+namespace N6 {
+  int &f(int);
+}
+
+namespace N7 {
+  int &f(int);
+}