[clang] [clang] Add the candiscard attribute to suppress nodiscard (PR #154943)

[via cfe-commits]

halbi2 wrote:

On the naming, I chose `candiscard` only because it seemed the antonym of `nodiscard`. It could be `discardable`, as `nodiscard` could be `nondiscardable`. No deeper meaning.

I don't know what an RFC is. Is it needed? What if this is "solution in search of a problem", what is the path to fix libc++ `expected` without adding this attribute?

https://github.com/llvm/llvm-project/pull/154943