jl_get_builtin_fptr not handled correctly for call resolution

[topolarity]

julia> foo() = Core.current_scope()
foo (generic function with 1 method)

julia> @code_llvm foo()
; Function Signature: foo()
;  @ REPL[3]:1 within `foo`
define nonnull ptr @julia_foo_2954() #0 {
top:
  %0 = call ptr @jl_get_builtin_fptr(ptr nonnull @"+Core.#current_scope#2956.jit")
  %Builtin_ret = call nonnull ptr %0(ptr nonnull @"jl_global#2957.jit", ptr null, i32 0)
  ret ptr %Builtin_ret
}

julia> check_allocs(foo, (); ignore_throw = false)
2-element Vector{Any}:
 Allocating runtime call to "jl_get_builtin_fptr" in ./REPL[3]:1
  | (source not available)
Stacktrace:
 [1] foo()
   @ Main ./REPL[3]:1

 Dynamic dispatch in ./REPL[3]:1
  | (source not available)
Stacktrace:
 [1] foo()
   @ Main ./REPL[3]:1

This should be a single runtime call (and it's also not allocating).

As part of fixing this, we should probably add an UnknownRuntimeFunction error type. That way we don't imply something is allocating when really we have an unresolved runtime call.