See https://github.com/llvm/llvm-project/pull/147168 for more info.

Support for translating the operations introduced in #144785 to LLVM-IR.

In order to keep the lowering simple,
`OpenMPIRBuider::unrollLoopHeuristic` is applied when encountering the
`omp.unroll_heuristic` op. As a result, the operation that unrolling is
applied to (`omp.canonical_loop`) must have been emitted before even
though logically there is no such requirement.

Eventually, all transformations on a loop must be applied directly after
emitting `omp.canonical_loop`, i.e. future transformations must be
looked-up when encountering `omp.canonical_loop` itself. This is because
many OpenMPIRBuilder methods (e.g. `createParallel`) expect all the
region code to be emitted withing a callback. In the case of
`createParallel`, the region code is getting outlined into a new
function. Therefore, making the operation order a formal requirement
would not make the implementation any easier.

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.

This patch makes a few changes to unify the conversion process from the
'omp' to the 'llvm' dialect. The main goal of this change is to
consolidate the logic used to identify legal and illegal ops, and to
consolidate the conversion logic into a single class.

Changes introduced are the following:
- Removal of `getNumVariableOperands()` and `getVariableOperand()` extra
class declarations from OpenMP operations. These are redundant, as they
are equivalent to `mlir::Operation::getNumOperands()` and
`mlir::Operation::getOperands()`, respectively.
- Consolidation of `RegionOpConversion`,
`RegionLessOpWithVarOperandsConversion`,
`RegionOpWithVarOperandsConversion`, `RegionLessOpConversion`,
`AtomicReadOpConversion`, `MapInfoOpConversion`,
`DeclMapperOpConversion` and `MultiRegionOpConversion` into a single
`OpenMPOpConversion` class. This is possible because all of the previous
were doing parts of the same set of operations based on whether they
defined any regions, whether they took operands, type attributes, etc.
- Update of `mlir::configureOpenMPToLLVMConversionLegality` to use a
single generic set of checks for all operations, removing the need to
list every operation manually.
- Update of `mlir::populateOpenMPToLLVMConversionPatterns` to
automatically populate the list of patterns to include all dialect operations.

Add conversion support from FIR to LLVM Dialect for OMP DeclareMapper.

Depends on #121001

The intention of this work is to give MLIR->LLVMIR conversion freedom to
control how the private variable is allocated so that it can be
allocated on the stack in ordinary cases or as part of a structure used
to give closure context for tasks which might outlive the current stack
frame. See RFC:

https://discourse.llvm.org/t/rfc-openmp-supporting-delayed-task-execution-with-firstprivate-variables/83084

For example, a privatizer for an integer used to look like
```mlir
  omp.private {type = private} @x.privatizer : !fir.ref<i32> alloc {
  ^bb0(%arg0: !fir.ref<i32>):
    %0 = ... allocate proper memory for the private clone ...
    omp.yield(%0 : !fir.ref<i32>)
  }
```

After this change, allocation become implicit in the operation:
```mlir
  omp.private {type = private} @x.privatizer : i32
```

For more complex types that require initialization after allocation, an
init region can be used:
``` mlir
  omp.private {type = private} @x.privatizer : !some.type init {
  ^bb0(%arg0: !some.pointer<!some.type>, %arg1: !some.pointer<!some.type>):
    // initialize %arg1, using %arg0 as a mold for allocations
    omp.yield(%arg1 : !some.pointer<!some.type>)
  } dealloc {
    ^bb0(%arg0: !some.pointer<!some.type>):
    ... deallocate memory allocated by the init region ...
    omp.yield
  }
```

This patch lays the groundwork for delayed task execution but is not
enough on its own.

After this patch all gfortran tests which previously passed still pass.
There
are the following changes to the Fujitsu test suite:
- 0380_0009 and 0435_0009 are fixed
- 0688_0041 now fails at runtime. This patch is testing firstprivate
variables with tasks. Previously we got lucky with the undefined
behavior and won the race. After these changes we no longer get lucky.
This patch lays the groundwork for a proper fix for this issue.

In flang the lowering re-uses the existing lowering used for reduction
init and dealloc regions.

In flang, before this patch we hit a TODO with the same wording when
generating the copy region for firstprivate polymorphic variables. After
this patch the box-like fir.class is passed by reference into the copy
region, leading to a different path that didn't hit that old TODO but
the generated code still didn't work so I added a new TODO in
DataSharingProcessor.

This commit add an NVIDIA-specific lowering of `cf.assert` to to
`__assertfail`.

Note: `getUniqueFormatGlobalName`, `getOrCreateFormatStringConstant` and
`getOrDefineFunction` are moved to `GPUOpsLowering.h`, so that they can
be reused.

Adds initial support for lowering the `loop` directive to MLIR.

The PR includes basic suport and testing for the following clauses:
 * `collapse`
 * `order`
 * `private`
 * `reduction`

Parent PR: #113911, only the latest commit is relevant to this PR.