"const" is extraneous in the presence of "constexpr" for simple
variables and arrays.

This patch replaces LLVM_ATTRIBUTE_UNUSED with [[maybe_unused]] where
we do not need to move the position of [[maybe_unused]] within
declarations.

Notes:

- [[maybe_unused]] is a standard feature of C++17.

- The compiler is far more lenient about the placement of
  __attribute__((unused)) than that of [[maybe_unused]].

I'll follow up with another patch to finish up the rest.

This update all of our Unicode tables to Unicode 15.1. This is a minor
version so only a relatively small numbers of characters are added,
mainly ideographs

https://www.unicode.org/versions/Unicode15.1.0/#Appendices_nb

startswith/endswith wrap starts_with/ends_with and will eventually go away (to more closely match string_view)

In a Unicode name was stored in a way that caused
a medial hyphen to be at the end of a a chunk, it would not
be properly ignored by the loose matching algorithm.

For example if `LEFT-TO-RIGHT OVERRIDE` was stored as
`LEFT-` [...], the `-` would not be ignored.

The generators now ensures nodes are not cut accross
medial hyphen boundaries.

Fixes #64161

Differential Revision: https://reviews.llvm.org/D156518

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

This patch mechanically replaces None with std::nullopt where the
compiler would warn if None were deprecated.  The intent is to reduce
the amount of manual work required in migrating from Optional to
std::optional.

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716