dillon1000/react

[compiler] Don't emit spurious `import { c as _c }` for discarded functions (#36500)

## What

Codegen registers `_c` (the memo cache import) as a side effect whenever
a function compiles with memo slots. The registration persists on
`ProgramContext.imports` even if the function is later discarded (`'use
no forget'`, `'use no memo'`, lint mode, validation errors). If other
applied functions in the file compile to 0 memo slots, the stale `import
{ c as _c } from "react/compiler-runtime";` leaks into the output.

## Fix

In `applyCompiledFunctions`, drop the memo cache import if no applied
function uses memo slots. If `react/compiler-runtime` has no remaining
specifiers, drop the module entry too so we don't emit a bare `import
"react/compiler-runtime";`.

## Reproducer

`use-no-forget-multiple-with-eslint-suppression.js`:

```js
import {useRef} from 'react';

const useControllableState = options => {};
function NoopComponent() {}

function Component() {
  'use no forget';
  const ref = useRef(null);
  // eslint-disable-next-line react-hooks/rules-of-hooks
  ref.current = 'bad';
  return <button ref={ref} />;
}
```

`NoopComponent` applies with 0 memo slots. `Component` is opted out, but
codegen already registered `_c` for it. Before:

```js
import { c as _c } from "react/compiler-runtime";
import { useRef } from "react";
```

After:

```js
import { useRef } from "react";
```

## Prior art

TS counterpart to the Rust port's fix in
7e26eb89fc3264a34a9f03a68ac7a870d9f74b51. That commit also added
`no-cache-slots-no-import.js`, which codifies the "no memo slots, no
import" rule.

## Test plan

- `yarn snap`: 1719/1719 passing
- Snapshot for `use-no-forget-multiple-with-eslint-suppression` loses
its spurious `_c` import
- `no-cache-slots-no-import` still passes