Revert "[Clang] Implement Change scope of lambda trailing-return-type"

This reverts commit d708a186 (and typo fix e4bc9898). It causes a compilation error for this:

```
struct StringLiteral {
  template <int N>
  StringLiteral(const char (&array)[N])
      __attribute__((enable_if(N > 0 && N == __builtin_strlen(array) + 1,
                               "invalid string literal")));
};

struct Message {
  Message(StringLiteral);
};

void Func1() {
  auto x = Message("x");  // Note: this is fine

  // Note: "xx\0" to force a different type, StringLiteral<3>, otherwise this
  // successfully builds.
  auto y = [&](decltype(Message("xx"))) {};

  // ^ fails with: repro.cc:18:13: error: reference to local variable 'array'
  // declared in enclosing function 'StringLiteral::StringLiteral<3>'

  (void)x;
  (void)y;
}
```

More details posted to D124351.