When OpenACC is enabled and Fortran loops are annotated with `acc loop`,
they are lowered to `acc.loop` operation. And rest of the contained
loops use the normal FIR lowering path.

Hovever, the OpenACC specification has special provisions related to
contained loops and their induction variable. In order to adhere to
this, we convert all valid contained loops to `acc.loop` in order to
store this information appropriately.

The provisions in the spec that motivated this change (line numbers are
from OpenACC 3.4):
- 1353 Loop variables in Fortran do statements within a compute
construct are predetermined to be private to the thread that executes
the loop.
- 3783 When do concurrent appears without a loop construct in a kernels
construct it is treated as if it is annotated with loop auto. If it
appears in a parallel construct or an accelerator routine then it is
treated as if it is annotated with loop independent.

By valid loops - we convert do loops and do concurrent loops which have
induction variable. Loops which are unstructured are not handled.

Reverts llvm/llvm-project#143441

Reverting due to CI failure
https://lab.llvm.org/buildbot/#/builders/53/builds/18055.

Atomic Control Options are used to specify architectural characteristics
to help lowering of atomic operations. The options used are:
`-f[no-]atomic-remote-memory`, `-f[no-]atomic-fine-grained-memory`,
 `-f[no-]atomic-ignore-denormal-mode`.
Legacy option `-m[no-]unsafe-fp-atomics` is aliased to
`-f[no-]ignore-denormal-mode`.
More details can be found in
https://github.com/llvm/llvm-project/pull/102569. This PR implements the
frontend support for these options with OpenMP atomic in flang.

Backend changes are available in the draft PR:
https://github.com/llvm/llvm-project/pull/143769 which will be raised
after this merged.

This patch fixes:

  flang/lib/Lower/Bridge.cpp:2128:10: error: unused variable 'result'
  [-Werror,-Wunused-variable]

Handle the case where the assigned variable also has a pointer
attribute.

Fixes #121721

Fixes #149089 and #149700.

Before #145837, when processing a reduction symbol not yet supported by
OpenMP lowering, the reduction processor would simply skip filling in
the reduction symbols and variables. With #145837, this behvaior was
slightly changed because the reduction symbols are populated before
invoking the reduction processor (this is more convenient to shared the
code with `do concurrent`).

This PR restores the previous behavior.

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

Pointer remappings unconditionally update the element byte size and
derived type of the pointer's descriptor. This is okay when the pointer
is polymorphic, but not when a pointer is associated with an extended
type.

To communicate this monomorphic case to the runtime, add a new entry
point so as to not break forward binary compatibility.

ArrayRef(std::nullopt_t) has been deprecated.  This patch replaces
std::nullopt with mlir::TypeRange{} or mlir::ValueRange{} as
appropriate.