PR46859: Fix crash if declaring a template in a DeclScope with no DeclContext.

This can happen during error recovery; it could also happen at block
scope if we ever parsed a template declaration at block scope.

clang/lib/Sema/SemaTemplate.cpp

@@ -7774,15 +7774,14 @@ Sema::CheckTemplateDeclScope(Scope *S, TemplateParameterList *TemplateParams) {
   // C++ [temp]p4:
   //   A template [...] shall not have C linkage.
   DeclContext *Ctx = S->getEntity();
-  assert(Ctx && "Unknown context");
-  if (Ctx->isExternCContext()) {
+  if (Ctx && Ctx->isExternCContext()) {
     Diag(TemplateParams->getTemplateLoc(), diag::err_template_linkage)
         << TemplateParams->getSourceRange();
     if (const LinkageSpecDecl *LSD = Ctx->getExternCContext())
       Diag(LSD->getExternLoc(), diag::note_extern_c_begins_here);
     return true;
   }
-  Ctx = Ctx->getRedeclContext();
+  Ctx = Ctx ? Ctx->getRedeclContext() : nullptr;
 
   // C++ [temp]p2:
   //   A template-declaration can appear only as a namespace scope or

clang/test/SemaCXX/extern-c.cpp

@@ -259,3 +259,8 @@ namespace PR35697 {
     }
   }
 }
+
+namespace PR46859 {
+  extern "bogus" // expected-error {{unknown linkage language}}
+    template<int> struct X {}; // expected-error {{templates can only be declared in namespace or class scope}}
+}