Now that `Typer.stabilize` ends up checking for `@uncheckedStable`
annotations to support defs's as stable paths, new opportunities
arise for cyclic errors.

This PR reduces the likelihood of cycles by typechecking the only core of
an annotation (_not_ the full application to the annotation arguments)
to determine its type symbol.

pos/unchecked-stable-cyclic-error.scala test case started to fail
since scala#8338 with:

```
|-- object Test BYVALmode-EXPRmode (site: package <empty>)
|    |-- super APPSELmode-EXPRmode-POLYmode-QUALmode (silent: <init> in Test)
|    |    |-- this EXPRmode (silent: <init> in Test)
|    |    |    \-> Test.type
|    |    \-> Test.type
|    |-- def foo BYVALmode-EXPRmode (site: object Test)
|    |    |-- attr EXPRmode (site: method foo in Test)
|    |    |    |-- Int TYPEmode (site: method attr in Test)
|    |    |    |    \-> Int
|    |    |    |-- new anno APPSELmode-BYVALmode-EXPRmode-FUNmode-POLYmode (site: object Test)
|    |    |    |    |-- new anno APPSELmode-EXPRmode-POLYmode-QUALmode (site: object Test)
|    |    |    |    |    |-- anno FUNmode-TYPEmode (site: object Test)
|    |    |    |    |    |    \-> anno
|    |    |    |    |    \-> anno
|    |    |    |    \-> (a: Any): anno
|    |    |    |-- (a: Any): anno : pt=anno EXPRmode (site: value <local Test> in Test)
|    |    |    |    |-- foo : pt=Any BYVALmode-EXPRmode (site: value <local Test> in Test)
|    |    |    |    |    caught scala.reflect.internal.Symbols$CyclicReference: illegal cyclic reference involving method foo: while typing foo
/Users/jz/code/scala/sandbox/test.scala:7: error: recursive method foo needs result type
  @anno(foo) def attr: Int = foo
        ^
|    |    |    |    \-> anno
```

Another variant, pos/annotation-cycle.scala, fails only on 2.12.x
(after the backport of scala#8338). Backporting this fix makes that test
start passing on 2.12.x.

Backports scala#9302