Revert "[Sema] Mark explicit specialization declaration in a friend invalid"

This reverts commit 8cb6ecbc4d.

Nothing wrong with the commit. It is missing Phabricator informations.

clang/lib/Sema/SemaDecl.cpp

@@ -9202,7 +9202,6 @@ Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
             << Name << RemoveRange
             << FixItHint::CreateRemoval(RemoveRange)
             << FixItHint::CreateInsertion(InsertLoc, "<>");
-          Invalid = true;
         }
       }
     } else {

clang/test/CXX/temp/temp.spec/temp.expl.spec/p20-2.cpp

@@ -1,15 +0,0 @@
-// RUN: %clang_cc1 -fsyntax-only -verify %s
-template<typename T>
-void f(T);
-
-template<typename T>
-struct A {
-  // expected-error@+1{{cannot declare an explicit specialization in a friend}}
-  template <> friend void f<>(int) {}
-};
-
-// Makes sure implicit instantiation here does not trigger
-// the assertion "Member specialization must be an explicit specialization"
-void foo(void) {
-    A<int> a;
-}