It is almost always simpler to use {} instead of std::nullopt to
initialize an empty ArrayRef. This patch changes all occurrences I could
find in LLVM itself. In future the ArrayRef(std::nullopt_t) constructor
could be deprecated or removed.

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

Normal compilation doesn't care about tracking references,
and shouldn't pay the compilation time cost.

Now that TableGen no longer relies on global Record state, we can allow
for the client to own the RecordKeeper and SourceMgr. Given that TableGen
internally still relies on the global llvm::SrcMgr, this method unfortunately
still isn't thread-safe.

Differential Revision: https://reviews.llvm.org/D125277

This commits removes TableGens reliance on managed static global record state
by moving the RecordContext into the RecordKeeper. The RecordKeeper is now
treated similarly to a (LLVM|MLIR|etc)Context object and is passed to static
construction functions. This is an important step forward in removing TableGens
reliance on global state, and in a followup will allow for users that parse tablegen
to parse multiple tablegen files without worrying about Record lifetime.

Differential Revision: https://reviews.llvm.org/D125276

This commit adds a new `TableGenParseFile` entry point for tablegen
that parses an input buffer and invokes a callback function with
a record keeper (notably without an output buffer). This kind of entry
point is very useful for tablegen consuming tools that don't create
output, and want invoke tablegen multiple times. The current way
that we interact with tablegen is via relative includes to
TGParser(not great).

Differential Revision: https://reviews.llvm.org/D119899