OpenMP 6.0 introduced a `fuse` directive, and with it a "loop sequence"
as the associated code. What used to be "loop association" has become
"loop-nest association".

Rename Association::Loop to LoopNest, add Association::LoopSeq to
represent the "loop sequence" association.

Change the association of fuse from "block" to "loop sequence".

Always rely on local scopes to enforce the lifetime of these helper
objects and by extension where the "closing" of various C++ code
constructs happens.

Emit empty line after a namespace scope is opened and before its closed.
Adjust DirectiveEmitter code empty line emission in response to this to
avoid lot of unit test changes.

Add a RAII class `IncludeGuardEmitter` which is similar to
`IfDefEmitter` but emits header include guards and adopt it in
DirectiveEmitter.

Additionally, add sentinel values <Enum>::First_ and <Enum>::Last_ to
each one of those enums.

This will allow using `enum_seq_inclusive` to generate the list of
enum-typed values of any generated scoped (non-bitmask) enum.

In "get<lang>DirectiveName(Kind, Version)", return the spelling that
corresponds to Version, and in "get<lang>DirectiveKindAndVersions(Name)"
return the pair {Kind, VersionRange}, where VersionRange contains the
minimum and the maximum versions that allow "Name" as a spelling. This
applies to clauses as well. In general it applies to classes that have
spellings (defined via TableGen class "Spelling").

Given a Kind and a Version, getting the corresponding spelling requires
a runtime search (which can fail in a general case). To avoid generating
the search function inline, a small additional component of
llvm/Frontent was added: LLVMFrontendDirective. The corresponding header
file also defines C++ classes "Spelling" and "VersionRange", which are
used in TableGen/DirectiveEmitter as well.

For background information see

https://discourse.llvm.org/t/rfc-alternative-spellings-of-openmp-directives/85507

Use the spellings in the generated clause parser. The functions
`get<lang>ClauseKind` and `get<lang>ClauseName` are not yet updated.

The definitions of both clauses and directives now take a list of
"Spelling"s instead of a single string. For example
```
def ACCC_Copyin : Clause<[Spelling<"copyin">,
                          Spelling<"present_or_copyin">,
                          Spelling<"pcopyin">]> { ... }
```

A "Spelling" is a versioned string, defaulting to "all versions".

For background information see

https://discourse.llvm.org/t/rfc-alternative-spellings-of-openmp-directives/85507