Add option and statement attribute for controlling emitting of
target-specific
metadata to atomicrmw instructions in IR.

The RFC for this attribute and option is

https://discourse.llvm.org/t/rfc-add-clang-atomic-control-options-and-pragmas/80641,
Originally a pragma was proposed, then it was changed to clang
attribute.

This attribute allows users to specify one, two, or all three options
and must be applied
to a compound statement. The attribute can also be nested, with inner
attributes
overriding the options specified by outer attributes or the target's
default
options. These options will then determine the target-specific metadata
added to atomic
instructions in the IR.

In addition to the attribute, three new compiler options are introduced:
`-f[no-]atomic-remote-memory`, `-f[no-]atomic-fine-grained-memory`,
 `-f[no-]atomic-ignore-denormal-mode`.
These compiler options allow users to override the default options
through the
Clang driver and front end. `-m[no-]unsafe-fp-atomics` is aliased to
`-f[no-]ignore-denormal-mode`.

In terms of implementation, the atomic attribute is represented in the
AST by the
existing AttributedStmt, with minimal changes to AST and Sema.

During code generation in Clang, the CodeGenModule maintains the current
atomic options,
which are used to emit the relevant metadata for atomic instructions.
RAII is used
to manage the saving and restoring of atomic options when entering
and exiting nested AttributedStmt.

`llvm::ConstantFP::get(llvm::LLVMContext&, APFloat(float))` always
returns a f32 constant.
Fix https://github.com/llvm/llvm-project/issues/107054.

Use this to replace the emission of the amdgpu-unsafe-fp-atomics
attribute in favor of per-instruction metadata. In the future
new fine grained controls should be introduced that also cover
the integer cases.

Add a wrapper around CreateAtomicRMW that appends the metadata,
and update a few use contexts to use it.