RFR: JDK-8265270: Type.getEnclosingType() may fail with CompletionFailure

[Jan Lahoda]

The setup is a little bit tricky, but it may happen that user code calls `Type{Mirror}.getEnclosingType()` (or e.g. `Type{Mirror}.toString()` which will in turn invoke `getEnclosingType()`), and the method may throw the `CompletionFailure` (internal) exception. The reason is that the corresponding Symbol is completed using a `complete` method, instead of `apiComplete` method, which is intended to be used in API methods. `apiComplete` does the same as `complete` when running in a javac context, but will suppress the exception when the API method was invoked from a user code, avoiding the exception.

The proposed change is to use `apiComplete`, as it should be done in implementations of API methods.

-------------

Commit messages:
 - JDK-8265270: Type.getEnclosingType() may fail with CompletionFailure

Changes: https://git.openjdk.java.net/jdk/pull/3512/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3512&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265270
  Stats: 61 lines in 2 files changed: 59 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3512.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3512/head:pull/3512

PR: https://git.openjdk.java.net/jdk/pull/3512