[AST] Replace assert with llvm_unreachable to silence compiler warning

New code added in ec3060c7 looked like

+  case TemplateName::NameKind::OverloadedTemplate:
+    assert(false && "overloaded templates shouldn't survive to here.");
+  default:

If compiling without asserts we then got a warning about unannotated
fallthrough from the case into the default.

Change the assert into an llvm_unreachable to silence the warning.