Allow specifying non-iterable return types for generators

[kdy1]

We should allow code like

stc/crates/stc_ts_type_checker/tests/conformance/types/asyncGenerators/types.asyncGenerators.es2018.1.ts

Lines 118 to 120 in 87d832a

	async function * explicitReturnType16(): {} {
	yield 1;
	}

Currently, stc fails with

  x SimpleAssignFailed {
  |     span: Span {
  |         lo: BytePos(
  |             3722,
  |         ),
  |         hi: BytePos(
  |             3729,
  |         ),
  |         ctxt: #0,
  |     },
  |     cause: Some(
  |         context: tried to get an element type from an async iterator for normal yield (at crates/stc_ts_file_analyzer/src/analyzer/stmt/return_type.rs:516:26)
  |         context: tried to get element of iterator as a fallback logic for async iterator (at crates/stc_ts_file_analyzer/src/analyzer/expr/array.rs:505:14)
  |         context: tried to get a type of an iterator to get the element type of it () (at crates/stc_ts_file_analyzer/src/analyzer/expr/array.rs:785:14)
  |         context: tried to get iterator (at crates/stc_ts_file_analyzer/src/analyzer/expr/array.rs:595:64)
  |         context: tried to convert a type ({
  |         };) to an iterator (at crates/stc_ts_file_analyzer/src/analyzer/expr/array.rs:760:13)
  |         context: tried to call `[Symbol.iterator]()` (at crates/stc_ts_file_analyzer/src/analyzer/expr/array.rs:757:14)
  |         context: tried to call a property of an object ({
  |         };) (at crates/stc_ts_file_analyzer/src/analyzer/expr/call_new.rs:969:10)
  |         context: failed to call property of type elements (at crates/stc_ts_file_analyzer/src/analyzer/expr/call_new.rs:1290:10)
  |         MustHaveSymbolAsyncIteratorThatReturnsIterator {
  |             span: Span {
  |                 lo: BytePos(
  |                     3722,
  |                 ),
  |                 hi: BytePos(
  |                     3729,
  |                 ),
  |                 ctxt: #0,
  |             },
  |         },
  |     ),
  | }
     ,-[$DIR/tests/conformance/types/asyncGenerators/types.asyncGenerators.es2018.1.ts:118:1]
 118 | async function * explicitReturnType16(): {} {
 119 |     yield 1;
     :     ^^^^^^^
 120 | }
     `----

But we should not report such errors if the return type is not iterable.