Increasing minimum required version of Clang to 17

[randombit]

Of our minimum supported compiler versions, LLVM Clang 14 is the most problematic. It's also I think our least important; it's only the default system compiler on BSDs.

[I forgot: Android NDK is also Clang, namely Clang 17]

Bumping minimum to Clang 17 gets us

• std::source_location
• P0634R3 is fixed
• Fixing several bugs in consteval I ran into over the summer
• Apparently some amount of ranges works (I haven't looked into this)
• std::format (but we'd still be blocked by GCC, since it wasn't added there until GCC 13)

#4528 contains proposed changes

The plan would be in 3.7.0 to have a configure time warning if an older Clang is detected, as well as a prominent mention in the release notes. Followed by merging #4528 for 3.8.0

Leave a comment here if removing support for Clang 14 through 16 would be an issue for you.

[Ashishbsharma]

Hi @randombit, I’d like to contribute to this issue by suggesting conditional fallbacks for features like std::source_location to ease the transition for users of older Clang versions while moving towards full adoption of Clang 17