AstGen: add result location pointers to function calls

[andrewrk]

const Foo = struct {
    x: i32,
    y: i32,
};

test {
    var x = foo();
}

fn foo() Foo {
    return .{ .x = 1, .y = 2 };
}

%27 = extended(struct_decl(parent, Auto, {
  [67] Foo line(0) hash(2de2fa0aaeae8e73185740a28fbfe332): %0 = block_inline({
    %1 = extended(struct_decl(parent, Auto, {}, {}, {
      x: @Ref.i32_type,
      y: @Ref.i32_type,
    }) node_offset:1:13
    %2 = break_inline(%0, %1)
  }) node_offset:1:1
  test line(5) hash(dbcecee70b63388d70a9ee646a22e98a): %3 = block_inline({
    %11 = extended(func(test, [], @Ref.void_type, inferror, {
      %4 = dbg_stmt(1, 4)
      %5 = alloc_inferred_mut() node_offset:7:5
      %6 = decl_val("foo") token_offset:7:13
      %7 = call(%6, []) node_offset:7:16
      %8 = store_to_inferred_ptr(%5, %7)
      %9 = resolve_inferred_alloc(%5) node_offset:7:5
      %10 = ret_coerce(@Ref.void_value) token_offset:8:1
    }) (lbrace=0:5,rbrace=2:0) node_offset:6:1
    %12 = break_inline(%3, %11)
  }) node_offset:6:1
  [81] foo line(9) hash(a123f81685e817c35bd084473236d80d): %13 = block_inline({
    %14 = decl_val("Foo") token_offset:10:10
    %15 = as_node(@Ref.type_type, %14) node_offset:10:10
    %25 = func([], %15, {
      %16 = dbg_stmt(1, 4)
      %17 = extended(ret_ptr()) node_offset:11:5
      %18 = field_ptr(%17, "x") node_offset:11:20
      %19 = store_node(%18, @Ref.one) node_offset:11:20
      %20 = field_ptr(%17, "y") node_offset:11:28
      %21 = int(2)
      %22 = store_node(%20, %21) node_offset:11:28
      %23 = validate_struct_init_ptr({
        %18 = field_ptr(%17, "x") node_offset:11:20
        %20 = field_ptr(%17, "y") node_offset:11:28
      }) node_offset:11:13
      %24 = ret_node(@Ref.void_value) node_offset:11:5
    }) (lbrace=0:13,rbrace=2:0) node_offset:10:1
    %26 = break_inline(%13, %25)
  }) node_offset:10:1
}, {}, {})

The problem is that the result location for x does not get passed to the function call:

// result location for `x`:
       %5 = alloc_inferred_mut() node_offset:7:5

// function call of `foo()`:
      %7 = call(%6, []) node_offset:7:16

Instead it is treated as returning a value. AstGen needs to be modified to pass the result location pointer to the function call, so the function can populate the result pointer.