Rename OptXXXEmitter.cpp to OptionXXXEmitter.cpp to have a less
ambiguous
name, as `Opt` could also mean optimization.

Move common functions shared by TableGen Option Emitter
and Options library to Support:
- Move `StrCmpOptionName` and base it on the existing version
  in OptTable.cpp, with an additional mode to control fall back to
  case insensitive comparison.
- Add `StrCmpOptionPrefixes` to compare prefixes and use zip() to
  iterate through lists of prefixes.
- Rename `CompareOptionRecords` to less ambiguous name 
  `IsOptionRecordLess`.
- Merge 2 back-to-back ifs with same condition in
  `IsOptionRecordLess`.

Fixes https://github.com/llvm/llvm-project/issues/107723

Migrate Opt/OptRST Emitters to const RecordKeeper.

This is a part of effort to have better const correctness in TableGen
backends:


https://discourse.llvm.org/t/psa-planned-changes-to-tablegen-getallderiveddefinitions-api-potential-downstream-breakages/81089

S.substr(N) is simpler than S.slice(N, StringRef::npos) and
S.slice(N, S.size()). Also, substr is probably better recognizable
than slice thanks to std::string_view::substr.

This reverts commit 67d20412b448533c77f54ac7a1e5d0775d273729.

This includes fixes for clanginstallapi.

This reverts commit 7e958f64efea6cb824e96ace51e021afbc150922.

Failing on multiple bots.

And use it to print the correct default OpenMP version for flang and
flang -fc1.

This change adds an optional `HelpTextsForVariants` to options. This
allows you to change the help text that gets shown in documentation and
`--help` based on the program its being generated for.

As `OptTable` needs to be constexpr compatible, I have used a std::array
of help text variants. Each entry is:
(list of visibilities) - > help text string

So for the OpenMP version we have (flang, fc1) -> "OpenMP version for
flang is...".

So you can have multiple visibilities use the same string. The number of
entries is currently set to 1, and the number of visibilities per entry
is 2, because that's the maximum we need for now. The code is written so
we can increase these numbers later, and the unused elements will be initialised.

I have not applied this to group descriptions just because I don't know
of one that needs changing. It could easily be enabled for those too if
needed. There are minor changes to them just to get it all to compile.

This approach of storing many help strings per option in the 1 driver
library seemed preferable to making a whole new library for Flang (even
if that would mostly be including stuff from Clang).

Refactor of the llvm-tblgen source into:
- a "Basic" library, which contains the bare minimum utilities to build
`llvm-min-tablegen`
- a "Common" library which contains all of the helpers for TableGen
backends. Such helpers can be shared by more than one backend, and even
unit tested (e.g. CodeExpander is, maybe we can add more over time)

Fixes #80647

These are unnecessary since C++17.

```
find llvm/utils/TableGen -iname "*.h" -o -iname "*.cpp" | xargs clang-format-16 -i
```

Split from #80847