stage2: implement tuples

* AIR instruction vector_init gains the ability to init arrays and
   tuples in addition to vectors. This will probably also gain the
   ability to initialize structs and be renamed to `aggregate_init`.
 * AstGen prefers to use an `anon_array_init` ZIR instruction for
   local variables when the init expr is an array literal and there is
   no type.

test/behavior/array_llvm.zig

@@ -237,8 +237,6 @@ test "zero-sized array with recursive type definition" {
 }
 
 test "type coercion of anon struct literal to array" {
-    if (@import("builtin").zig_backend == .stage2_llvm) return error.SkipZigTest; // TODO
-
     const S = struct {
         const U = union {
             a: u32,
@@ -254,6 +252,8 @@ test "type coercion of anon struct literal to array" {
             try expect(arr1[1] == 56);
             try expect(arr1[2] == 54);
 
+            if (@import("builtin").zig_backend == .stage2_llvm) return error.SkipZigTest; // TODO
+
             var x2: U = .{ .a = 42 };
             const t2 = .{ x2, .{ .b = true }, .{ .c = "hello" } };
             var arr2: [3]U = t2;