[llvm] [ADT] Add and use (for AArch64) `ValueWithSentinel<T, Sentinel>` (PR #158120)

[Kazu Hirata via llvm-commits]

https://github.com/kazutakahirata commented:

I'm a bit ambivalent about this PR.  I like the safety aspect of `ValueWithSentinel`, but then I wonder if `ValueWithSentinel` makes code less readable than household names like `std::optional`.

How about naming the new class `OptionalWithSentinel` or something?  This way, people might think of connections with `std::optional` in the back of their mind.

By the way, you might want to check to see if `CustomizableOptional` in `clang/Basic/CustomizableOptional.h` fits your bill.  If so, feel free to move it back to `llvm/include/ADT`.

This class lets you define your own `OptionalStorage` class.  Your `OptionalStorage` is responsible for constructors and `has_value`.  IIUC, `CustomizableOptional<DirectoryEntryRef>` uses a trick similar to `std::numeric_limits<T>::max()` to avoid allocating a separate boolean value.  `CustomizableOptional` handles the rest (`operator*`, `operator==`, etc).  A quick history is that we used to have our own implementation of `std::optional` called `llvm::Optional`.  `llvm::Optional` had this customizable storage capability.  Since `clang` was the only user of the customizable storage, we moved it to `clang/include/clang/Basic` while the rest of the codebase uses the standard `std::optional`.


https://github.com/llvm/llvm-project/pull/158120