Use result location type in unionInit

[silversquirl]

@unionInit could infer its target type from the result location, similar to #5909.

// Old
const foo = @unionInit(MyUnion, "field", value);
// New
const foo: MyUnion = @unionInit("field", value);

// Old
const foo: MyStruct = .{ .my_field = @unionInit(MyUnion, "field", value) };
// New
const foo: MyStruct = .{ .my_field = @unionInit("field", value) };

// Old
myFunc(@unionInit(MyUnion, "field", value));
// New
myFunc(@unionInit("field", value));