[clang] [clang][Sema] Add -Wswitch-default warning option (PR #73077)

[Erich Keane via cfe-commits]

erichkeane wrote:

I think there is value to adding this if only for the GCC compat story here.  I realize we don't like non-on-by-default warnings, but it IS something that folks use reasonably often, and we've even acquiesced to making it an ignored flag IIRC.  Aaron is the real decision maker here, but I think I'd have no problem holding my nose and allowing the non-on-by-default here, if only to match GCC.

BUT, this should make sure to match behavior of GCC WRT enums/etc.  But if folks want a warning for a fully-covered enum, I think there is value in giving it to them.

Patch itself need some additional testing (particularly around templates/dependence/etc), a release note, additional documentation in our commandline reference/etc.



https://github.com/llvm/llvm-project/pull/73077