CompilerId: Restore support for classic C by avoiding C++ style comments

C++ style comments were added by commit fc3a1cbd (CompilerID: Compiler
extensions default detection, 2021-05-29, v3.22.0-rc1~52^2~2), but they
may not be supported by the default mode of some C compilers.  Use
C-style comments instead.  For consistency, do this for all languages.

Fixes: #22942
49 jobs for !6759 with compiler-id-classic-c in 12 minutes and 52 seconds (queued for 2 seconds)
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