[Sema] diagnoseEquivalentInternalLinkageDeclarations - assert for non-null NamedDecl. NFCI.

Fixes clang static analysis warnings.

Author: RKSimon

clang/lib/Sema/SemaOverload.cpp

@@ -9926,6 +9926,7 @@ bool Sema::isEquivalentInternalLinkageDeclaration(const NamedDecl *A,
 
 void Sema::diagnoseEquivalentInternalLinkageDeclarations(
     SourceLocation Loc, const NamedDecl *D, ArrayRef<const NamedDecl *> Equiv) {
+  assert(D && "Unknown declaration");
   Diag(Loc, diag::ext_equivalent_internal_linkage_decl_in_modules) << D;
 
   Module *M = getOwningModule(D);