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

[via cfe-commits]

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+! RUN: %flang %s 2>&1 | FileCheck --check-prefix=STRICT %s
+! RUN: %flang -fno-system-clock-strict %s 2>&1 | FileCheck --allow-empty --implicit-check-not="SYSTEM_CLOCK" %s
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chris-earl wrote:

Done.

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