RFR: 8194743: Compiler implementation for Statements before super() [v5]

[Archie Cobbs]

> This is a first draft of a patch for JEP 447.
> 
> Summary of changes:
> 
> 1. Track when we're within a constructor "prologue" via new flag `AttrContext.ctorPrologue`
> 1. Add checks for illegal early access to `this` in constructor prologues, and update existing checks to distinguish between static context vs. constructor prologue context
> 1. Verify allowed placement of `super()`/`this()` calls via new method `Check.checkSuperInitCalls()`
> 1. Remove/refactor assumptions in several places that `super()`/`this()` was always the first statement
> 
> The changes in `Flow.java` are an example of <span>#</span>4. `Flow.FlowAnalyzer` checks for uncaught checked exceptions. For initializer blocks, this was previously done by requiring that any checked exceptions thrown be declared as thrown by all constructors containing `super()`. This list of checked exceptions was being pre-calculated before recursing into the initial constructors. This worked because initializer blocks were executed at the beginning of each initial constructor right after `super()` is called.
> 
> Now initializer blocks are traversed as each `super()` invocation is encountered, reflecting what actually happens at runtime. Similarly, final fields are marked as DA after encountering `this()`, not automatically at the beginning of those  constructors. These changes produce equivalent checks, but are compatible with the new flexibility of placement of `super()`/`this()` as well as possible future changes that could occur along these same lines.

Archie Cobbs has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 22 commits:

 - Rename unit test to be consistent with other feature exampless.
 - Update unit test after merged-in commit eaa80ad08.
 - Add unit tests with local class decl's prior to super().
 - Merge branch 'master' into SuperInit
 - Use @enablePreview in tests in preference to explicit command line flags.
 - Make "statements before super()" support a preview feature.
   
   Thanks to Jim Laskey for help with preview logic.
 - Small refactoring to avoid redundant test.
 - Fix typo in comment.
 - Use for() loop instead of stream for efficiency.
 - Fix typo in comment.
 - ... and 12 more: https://git.openjdk.org/jdk/compare/bddf4838...543a596a

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Changes: https://git.openjdk.org/jdk/pull/13656/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13656&range=04
  Stats: 1402 lines in 24 files changed: 1132 ins; 163 del; 107 mod
  Patch: https://git.openjdk.org/jdk/pull/13656.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13656/head:pull/13656

PR: https://git.openjdk.org/jdk/pull/13656