[clang] [flang] [flang] Add checks for Fortran 2023 system clock and add f2023/f202Y to valid arguments of -std= (PR #205938)

[via cfe-commits]

chris-earl wrote:

@cenewcombe @tarunprabhu @mleair Thank you for your feedback! I believe I have addressed everything brought up so far with my latest update.

Other than minor fixes, the latest version has removed the `-f{no-}system-clock-strict` flag in favor of the `-W{no-}system-clock-strict` flag. The `-f` flag version was added when I thought this change would include compile-time errors. Since the community has decided that the `SYSTEM_CLOCK` argument checks should be warnings only, the `-f` flag version is unnecessary and confusing.

Please let me know if you have any other feedback or questions, or if I missed anything.

https://github.com/llvm/llvm-project/pull/205938