Node v14 can’t build with clang 16 due to `-Wenum-constexpr-conversion`
errors. Since the backport patch from v8 does not apply to Node v14, and
it is likely this will become a hard error in future versions of clang,
use clang 15 when the version in the stdenv is newer.
The version of libc++ used with the clang is made to match the one used
in the stdenv to avoid possible issues with mixing multiple versions of
libc++ in one binary (e.g., icu links against libc++).
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