improvements to @asyncCall

* `await @asyncCall` generates better code. See #3065
 * `@asyncCall` works with a real `@Frame(func)` in addition to
   a byte slice. Closes #3072
 * `@asyncCall` allows passing `{}` (a void value) as the result
   pointer, which uses the result location inside the frame.
   Closes #3068
 * support `await @asyncCall` on a non-async function. This is in
   preparation for safe recursion (#1006).

test/compile_errors.zig

@@ -2,6 +2,22 @@ const tests = @import("tests.zig");
 const builtin = @import("builtin");
 
 pub fn addCases(cases: *tests.CompileErrorContext) void {
+    cases.add(
+        "wrong type for result ptr to @asyncCall",
+        \\export fn entry() void {
+        \\    _ = async amain();
+        \\}
+        \\fn amain() i32 {
+        \\    var frame: @Frame(foo) = undefined;
+        \\    return await @asyncCall(&frame, false, foo);
+        \\}
+        \\fn foo() i32 {
+        \\    return 1234;
+        \\}
+    ,
+        "tmp.zig:6:37: error: expected type '*i32', found 'bool'",
+    );
+
     cases.add(
         "struct depends on itself via optional field",
         \\const LhsExpr = struct {