Avoid extra messages if a template is declared in wrong place.

[sepavloff]

Summary

If a template is declared in a scope, where it may not, do not mark it
as invalid declaration. Name lookup then can find the declaration, this
reduce number of diagnostic messages.

For instance, the code:

extern "C" {
  namespace ns11416 {
    template <class R> class Sphere_d;
    template <class R> bool equal_as_sets(const Sphere_d<R>&, const Sphere_d<R>&);
  }
}

produces:

pr11416.cpp:3:5: error: templates must have C++ linkage
    template <class R> class Sphere_d;
    ^~~~~~~~~~~~~~~~~~
pr11416.cpp:4:49: error: unknown type name 'Sphere_d'
    template <class R> bool equal_as_sets(const Sphere_d<R>&, const Sphere_d<R>&);
                                                ^
pr11416.cpp:4:57: error: expected ')'
    template <class R> bool equal_as_sets(const Sphere_d<R>&, const Sphere_d<R>&);
                                                        ^
pr11416.cpp:4:42: note: to match this '('
    template <class R> bool equal_as_sets(const Sphere_d<R>&, const Sphere_d<R>&);
                                         ^
pr11416.cpp:4:5: error: templates must have C++ linkage
    template <class R> bool equal_as_sets(const Sphere_d<R>&, const Sphere_d<R>&);
    ^~~~~~~~~~~~~~~~~~

With this patch only messages 'templates must have C++ linkage' are produced.
GCC also does not emit extra messages.

[sepavloff]

Sending the patch...

[sepavloff]

There must be more general way to cope with this problem.

lib/Sema/SemaTemplate.cpp

Sema::CheckClassTemplate(Scope *S, unsigned TagSpec, TagUseKind TUK,
                          unsigned NumOuterTemplateParamLists,
                          TemplateParameterList** OuterTemplateParamLists) {
   assert(TemplateParams && TemplateParams->size() > 0 &&
          "No template parameters");
   assert(TUK != TUK_Reference && "Can only declare or define class templates");
   bool Invalid = false;
 
   // Check that we can declare a template here.
-  if (CheckTemplateDeclScope(S, TemplateParams))
-    return true;
+  CheckTemplateDeclScope(S, TemplateParams);
 
   TagTypeKind Kind = TypeWithKeyword::getTagTypeKindForTypeSpec(TagSpec);
   assert(Kind != TTK_Enum && "can't build template of enumerated type");
 
   // There is no such thing as an unnamed class template.
   if (!Name) {
     Diag(KWLoc, diag::err_template_unnamed_class);
     return true;

test/SemaTemplate/class-template-decl.cpp

namespace redecl {
   };
 }
 
 extern "C" template <typename T> // expected-error{{templates must have C++ linkage}}
 void DontCrashOnThis() {
   T &pT = T();
   pT;
 }
+
+extern "C" {
+  namespace ns11416 {
+    template <class R> class Sphere_d;  //expected-error{{templates must have C++ linkage}}
+    template <class R> bool equal_as_sets(const Sphere_d<R>&, const Sphere_d<R>&);  //expected-error{{templates must have C++ linkage}}
+  }
+}