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".

The check of whether a construct can privatize is a general utility, not
dependent on language-specific definitions.

Co-authored-by: Michael Klemm <michael.klemm@amd.com>

Add 3.1 and 4.0 as versions. This will make flang's default OpenMP
version (3.1) be included in the list.

The main issue to solve is that OpenMP modifiers can be specified in any
order, so the parser cannot expect any specific modifier at a given
position. To solve that, define modifier to be a union of all allowable
specific modifiers for a given clause.

Additionally, implement modifier descriptors: for each modifier the
corresponding descriptor contains a set of properties of the modifier
that allow a common set of semantic checks. Start with the syntactic
properties defined in the spec: Required, Unique, Exclusive, Ultimate,
and implement common checks to verify each of them.

OpenMP modifier overhaul: #2/3

Identified with misc-include-cleaner.

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.

The kernel names for OpenMP are manually mangled and not ideal when we
report something to the user. We demangle them now, providing the
function and line number of the target region, together with the actual
kernel name.

This function will break up a construct into constituent leaf and
composite constructs, e.g. if OMPD_c_d_e and OMPD_d_e are composite
constructs, then OMPD_a_b_c_d_e will be broken up into the list {OMPD_a,
OMPD_b, OMPD_c_d_e}.

Implement helper functions to identify leaf, composite, and combined
constructs.