LLVM: Activate typed pointers through the API instead of a global option.

[maleadt]

This allows other users of LLVM to use opaque pointers on their contexts. I'd like to get this in 1.10 (or a minor version) so that we can start updating packages to be compatible with opaque pointers. Currently, that requires overriding the global option, which is unwanted (may break Julia, definitely breaks certain GPUCompiler back-ends, etc).

Picked from #49846.

[maleadt]

@vchuravy I did just realize that this change results in LLVM's default Context() using opaque pointers by default now. Which means that using that in generated functions results in incompatible IR:

julia> using LLVM, LLVM.Interop

julia> @generated function memset(ptr::Ptr{T}, val::T) where T
           @dispose ctx=Context() begin
               T_ptr = convert(LLVMType, Ptr{T})
               T_val = convert(LLVMType, T)
               T_actual_ptr = LLVM.PointerType(T_val, 0)
               f, ft = create_function(LLVM.VoidType(), [T_ptr, T_val])

               @dispose builder=IRBuilder() begin
                   entry = BasicBlock(f, "entry")
                   position!(builder, entry)
                   actual_ptr = inttoptr!(builder, parameters(f)[1], T_actual_ptr)
                   store!(builder, parameters(f)[2], actual_ptr)
                   ret!(builder)
               end

               call_function(f, Nothing, Tuple{Ptr{T}, T}, :ptr, :val)
           end
       end
memset (generic function with 1 method)

julia> a = [0]
1-element Vector{Int64}:
 0

julia> memset(pointer(a), 42)
<string>:7:27: warning: ptr type is only supported in -opaque-pointers mode
  %2 = inttoptr i64 %0 to ptr
                          ^
ERROR: Failed to parse LLVM assembly:
<string>:7:27: error: expected type
  %2 = inttoptr i64 %0 to ptr
                          ^

Stacktrace:
 [1] macro expansion
   @ Main ~/Julia/pkg/LLVM/src/interop/base.jl:38 [inlined]
 [2] macro expansion
   @ Main ./REPL[2]:1 [inlined]
 [3] memset(ptr::Ptr{Int64}, val::Int64)
   @ Main ./REPL[2]:1
 [4] top-level scope
   @ REPL[4]:1

We also can't have LLVM.jl default to Context() disabling opaque pointers, because you can only set the option once...

---

EDIT: as noted by @vtjnash, the reverse (using typed IR in an opaque context) is allowed, so we should just default to using typed contexts until we upgrade to LLVM 17.