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

[Aaron Ballman via cfe-commits]

AaronBallman 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.

I think GCC compatibility is sufficient reason to support this despite being off-by-default and basically no hope of ever enabling it by default. There's evidence that people enable this in the wild, so I think it meets the bar: https://sourcegraph.com/search?q=context%3Aglobal+-Wswitch-default+-file%3A.*clang.*+-file%3A.*gcc.*&patternType=standard&sm=1&groupBy=repo

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