Test against FileCheck from LLVM 12 and 11

[rathann]

Similar to #100, please switch to testing against recent LLVM. Versions 8 and 9 are long obsolete and no longer included in modern Fedora. Additional Fedora LLVM package maintainer's policy is to include the full build only for the latest version, so FileCheck is not included with older releases (see 1632732 1939414) so I have no way of running the tests on non-x86_64 architectures and I'm forced to patch it out.