Note that llvm::interleaved constructs a string with the elements from
a given range with a given separator.

These are identified by misc-include-cleaner.  I've filtered out those
that break builds.  Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.

Now that we have a dedicated abstraction for string tables, switch the
option parser library's string table over to it rather than using a raw
`const char*`. Also try to use the `StringTable::Offset` type rather
than a raw `unsigned` where we can to avoid accidental increments or
other issues.

This is based on review feedback for the initial switch of options to a
string table. Happy to tweak or adjust if desired here.

Apologies for the large change, I looked for ways to break this up and
all of the ones I saw added real complexity. This change focuses on the
option's prefixed names and the array of prefixes. These are present in
every option and the dominant source of dynamic relocations for PIE or
PIC users of LLVM and Clang tooling. In some cases, 100s or 1000s of
them for the Clang driver which has a huge number of options.

This PR addresses this by building a string table and a prefixes table
that can be referenced with indices rather than pointers that require
dynamic relocations. This removes almost 7k dynmaic relocations from the
`clang` binary, roughly 8% of the remaining dynmaic relocations outside
of vtables. For busy-boxing use cases where many different option tables
are linked into the same binary, the savings add up a bit more.

The string table is a straightforward mechanism, but the prefixes
required some subtlety. They are encoded in a Pascal-string fashion with
a size followed by a sequence of offsets. This works relatively well for
the small realistic prefixes arrays in use.

Lots of code has to change in order to land this though: both all the
option library code has to be updated to use the string table and
prefixes table, and all the users of the options library have to be
updated to correctly instantiate the objects.

Some follow-up patches in the works to provide an abstraction for this
style of code, and to start using the same technique for some of the
other strings here now that the infrastructure is in place.

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.

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

This reverts commit 67d20412b448533c77f54ac7a1e5d0775d273729.

This includes fixes for clanginstallapi.

This reverts commit 7e958f64efea6cb824e96ace51e021afbc150922.

Failing on multiple bots.