RFR: Add alternate desugaring strategy for null-restricted arrays [v3]

[Maurizio Cimadamore]

> Creation of null-restricted arrays are desugared as calls into methods of the `ArrayCreation` class, as we need to set the "strict" marker on the created array.
> Currently the JVM doesn't support creation of null-restricted array for non-value types, so in this PR I cleaned up the `ArrayCreation` API to reflect more what javac needs. Also, according to our long term plans, `ArrayCreation` is really implemented on top of a new primitive in `Array`, which will be refined later to support creation of arrays with desired flags.
> 
> The translation strategy in javac is simple -- given an array initializer `{ .. }`, we just assign the initializer to a temp variable, then wrap the initializer using a call to `ArrayCreation::copied` to obtain the new array. Then we wrap everything in a let expression, so that things compose nicely.
> 
> While writing the test I realized that our combo framework failed to report compilation and execution failures. As I fixed that issue, I realized that a test I've added (`NullRestrictionReflectiveTest`) was not working correctly, as it had compilaton issues with some of the combinations and even a latent issue in signature parsing which I have now fixed.

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Address review comments

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Changes:
  - all: https://git.openjdk.org/valhalla/pull/1877/files
  - new: https://git.openjdk.org/valhalla/pull/1877/files/0bf345a0..91b152e3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1877&range=02
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1877&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/valhalla/pull/1877.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1877/head:pull/1877

PR: https://git.openjdk.org/valhalla/pull/1877