[MC] Remove SMRange(std::nullopt_t) (#165832)
This patch removes SMRange(std::nullopt_t) to reduce the number of
uses of std::nullopt outside the context of std::optional. Since
there are only a handful of uses, this patch removes the constructor
without going through deprecation.
The use of std::nullopt here has its root in llvm::None, which was
used as a convenient way to indicate "nothing" before we migrated
llvm::Optional to std::optional.
