Itanium ABI: Properly mangle extern "C" template arguments

[majnemer]

Summary

extern "C" declarations should be considered like global declarations
for mangling purposes.

[rsmith]

LGTM

[yaron.keren]

There is another copy of getEffectiveDeclContext in MicrosoftMangle.cpp which pre-commit was identical to this one.
Should it get this commit as well to keep both identical?
If so, could both copies of getEffectiveDeclContext be merged?

lib/AST/ItaniumMangle.cpp

if (ParmVarDecl *ContextParam
           = dyn_cast_or_null<ParmVarDecl>(BD->getBlockManglingContextDecl()))
       return ContextParam->getDeclContext();
   }
   
   const DeclContext *DC = D->getDeclContext();
   if (const CapturedDecl *CD = dyn_cast<CapturedDecl>(DC))
     return getEffectiveDeclContext(CD);
 
+  if (const auto *VD = dyn_cast<VarDecl>(D))
+    if (VD->isExternC())
+      return VD->getASTContext().getTranslationUnitDecl();
+
+  if (const auto *FD = dyn_cast<FunctionDecl>(D))
+    if (FD->isExternC())
+      return FD->getASTContext().getTranslationUnitDecl();
+
   return DC;
 }
 
 static const DeclContext *getEffectiveParentContext(const DeclContext *DC) {
   return getEffectiveDeclContext(cast<Decl>(DC));
 }
 
 static bool isLocalContainerContext(const DeclContext *DC) {

test/CodeGenCXX/mangle-template.cpp

 // CHECK: void @_ZN5test22f1ENS_2t1IXadL_ZNS_2f0EfEEEE(
 void f1(t1<f0> a0) {}
 }
 
 namespace test3 {
 // CHECK: void @test3_f0
 extern "C" void test3_f0(float) {}
 template<void (&)(float)> struct t1 {};
-// FIXME: Fails because we tack on a namespace.
-// CHECK-FIXME: void @_ZN5test32f1ENS_2t1ILZ8test3_f0EEE(
+// CHECK: void @_ZN5test32f1ENS_2t1IL_Z8test3_f0EEE(
 void f1(t1<test3_f0> a0) {}
 }
 
 namespace test4 {
 // CHECK: void @test4_f0
 extern "C" void test4_f0(float) {}
 template<void (*)(float)> struct t1 {};
-// CHECK: void @_ZN5test42f1ENS_2t1IXadL_ZNS_8test4_f0EEEEE(
+// CHECK: void @_ZN5test42f1ENS_2t1IXadL_Z8test4_f0EEEE(
 void f1(t1<test4_f0> a0) {}
 }
 
 // CHECK: void @test5_f0
 extern "C" void test5_f0(float) {}
 int main(int) {}
 
 namespace test5 {