This patch upstreams CIR support for atomic operations with memory
orders that are not known at compile time.

This patch adds CIR support for atomic fetch-and-update operations,
including the intrinsic functions `__atomic_fetch_<binop>`,
`__atomic_<binop>_fetch`, and `__c11_atomic_fetch_<binop>`, where
`<binop>` could be `add`, `sub`, `max`, `min`, `and`, `or`, `xor`, and
`nand`.

This patch adds support for the following atomic builtin functions:
  - `__atomic_test_and_set`
  - `__atomic_clear`

This patch updates the definitions of `cir.atomic.xchg` and
`cir.atomic.cmpxchg` to make them follow the established CIR assembly
conventions. Some other minor changes are also made along the way:

- The verifier for `cir.atomic.cmpxchg` is now fully declared in
TableGen.
- The `Op` suffix is appended to `CIR_AtomicXchg` and
`CIR_AtomicCmpXchg` to follow the naming conventions for TableGen
operation records.

Implement emitAtomicInit support for AggregateExpr

This patch adds atomic exchange operation which covers the following
C/C++ intrinsic functions:

  - `__c11_atomic_exchange`
  - `__atomic_exchange`
  - `__atomic_exchange_n`

This patch adds support for atomic compare-and-swap operations,
including the following C/C++ instrinsics:

  - `__atomic_compare_exchange`
  - `__atomic_compare_exchange_n`
  - `__c11_atomic_compare_exchange_strong`
  - `__c11_atomic_compare_exchange_weak`

This change adds support for Atomic init for ComplexType

Issue: https://github.com/llvm/llvm-project/issues/141365

This patch adds support for `__atomic_load_n` and `__atomic_store_n`
that were missed by #153814.