Support for fil-c

[pmasschelier]

fil-c is a fork of clang with memory safety in mind.
While it can be treated exactly as clang for almost everything, its version string is a bit different because it prepend Fil-c {FIL_C_VERSION} to the clang version string which makes meson use this version thus disabling C23 support for instance.

[dcbaker]

If anyone is interested in this, the result will probably look very similar to #15169, which implements a GCC derived compiler.