The CXX module initializer function, which is called at program startup,
needs to be tagged with Pointer Authentication and Branch Target
Identification marks whenever relevant.

Before this patch, in CPUs set up for PACBTI execution, the function
wasn't protected with return address signing and no BTI instruction was
inserted at the start of it, thus leading to an execution fault.

This patch fixes the issue by marking the function with the function
attributes related to PAC and BTI if relevant.

This is an alternative to
https://github.com/llvm/llvm-project/pull/122103

In SPIR-V, private global variables have the Private storage class. This
PR adds a new address space which allows frontend to emit variable with
this storage class when targeting this backend.

This is covered in this proposal: llvm/wg-hlsl@4c9e11a

This PR will cause addrspacecast to show up in several cases, like class
member functions or assignment. Those will have to be handled in the
backend later on, particularly to fixup pointer storage classes in some
functions.

Before this change, global variable were emitted with the 'Function'
storage class, which was wrong.

This is an expensive header, only include it where needed. Move some
functions out of line to achieve that.

This reduces time to build clang by ~0.5% in terms of instructions
retired.

Destructor calls were emitted without convergence intrinsics when building for SPIR-V, which means invalid IR since we
mixed controlled and non-controlled convergence.

We can use *Set::insert_range to collapse:

  for (auto Elem : Range)
    Set.insert(E);

down to:

  Set.insert_range(Range);

In some cases, we can further fold that into the set declaration.

Fixes #114126

Follow-up to https://github.com/llvm/llvm-project/issues/123569

Create separate resource initialization function for each resource and
add them to CodeGenModule's `CXXGlobalInits` list.
Fixes #120636 and addresses this [comment
](https://github.com/llvm/llvm-project/pull/119755/files#r1894093603).