Currently there is no way to assert that a check does not produce warnings for
a specific run. The CHECK-NOT directive can be used, but an empty file is
always discarded by FileCheck. Extend the FileCheck invocation with
'--allow-empty' to support this use-case.
This allows the following usage:
// RUN: %check_clang_tidy %s -check-suffix=CUSTOM <check-name> %t // CHECK-NOTES-CUSTOM-NOT: NO DIAGS