Fix crash in cleanup attr handling

ResolveSingleFunctionTemplateSpecialization() returns 0 and doesn't emit diags
unless the expression has template-ids, so we must null check the result.

Also add a better diag noting which overloads are causing the problem.

Reviewed by Aaron Ballman.

llvm-svn: 193055

clang/lib/Sema/SemaOverload.cpp

@@ -9672,6 +9672,9 @@ Sema::ResolveAddressOfOverloadedFunction(Expr *AddressOfExpr,
 /// template, where that template-id refers to a single template whose template
 /// arguments are either provided by the template-id or have defaults,
 /// as described in C++0x [temp.arg.explicit]p3.
+///
+/// If no template-ids are found, no diagnostics are emitted and NULL is
+/// returned.
 FunctionDecl *
 Sema::ResolveSingleFunctionTemplateSpecialization(OverloadExpr *ovl, 
                                                   bool Complain,