Consider updating Clang to 15.0.7

[SwooshyCueb]

We currently use Clang 13.0.1, which has a number of problems we have to work around, including but not limited to:

• Intermittent crashes when compiling C code containing variable-length arrays
• consteval is broken
• Incompatible with libstdc++'s coroutines

I think it makes sense to break our package version freeze to update Clang, as when we transition to using distro-provided compilers, we will almost certainly be transitioning to a version newer than 13, and updating our Clang package will make that transition easier.

Here's a table of the versions of Clang and GCC available on the distributions we support:

Distribution	Distro EOL	Available Clang versions (default emphasized)	Available GCC versions (default emphasized)
Ubuntu 20.04	2025-04	6.0.1, 7.0.1, 8.0.1, 9.0.1, 10.0.0, 11.0.0, 12.0.0	3.3.6, 7.5.0, 8.4.0, 9.4.0, 10.5.0
Ubuntu 22.04	2027-04	11.1.0, 12.0.1, 13.0.1, 14.0.0, 15.0.7	3.3.6, 9.5.0, 10.5.0, 11.4.0, 12.3.0
Ubuntu 24.04	2029-04	14.0.6, 15.0.7, 16.0.6, 17.0.6, 18.1.3	3.3.6, 9.5.0, 10.5.0, 11.4.0, 12.3.0, 13.2.0, 14.2.0
Debian 11	2026-08-31	9.0.1, 11.0.1, 13.0.1, 16.0.6	3.3.6, 9.3.0, 10.2.1
Debian 12	2028-06-30	13.0.1, 14.0.6, 15.0.6, 16.0.6	3.3.6, 11.3.0, 12.2.0
EL8	2029-05-31	17.0.6, 18.1.8	8.5.0, 9.2.1, 10.3.1, 11.2.1, 12.2.1, 13.3.1, 14.2.1
EL9	2032-05-31	18.1.8	11.5.0, 12.2.1, 13.3.1, 14.2.1

Ignoring Ubuntu 20.04 (as we are unlikely to transition to distro-provided compilers before April), I think updating to Clang 15 is the best candidate for helping to ease the transition to distro-provided compilers.
Unfortunately, we will still need to build Clang with GCC 10 (11 on Debian 12 and EL9) as the latest version of libstdc++ available on Debian 11 is 10.2.1.

I don't consider this a super high priority at the moment, but would love to hear people's thoughts.

Also, a quick note on the transition to distro-provided compilers: While the default GCC (and therefore the default set of libstdc++ headers) for EL8 is 8.5, EL8's clang is actually built with GCC 13, meaning we won't have to worry about EL8's Clang defaulting to a relatively ancient version of libstdc++.

[trel]

makes sense to me.

[korydraughn]

I agree.