RFR: 8317376: Minor improvements to the 'this' escape analyzer [v5]

[Archie Cobbs]

> Please review several fixes and improvements to the `this-escape` lint warning analyzer.
> 
> The goal here is to apply some relatively simple logical fixes that improve the precision and accuracy of the analyzer, and capture the remaining low-hanging fruit so we can consider the analyzer relatively complete with respect to what's feasible with its current design.
> 
> Although the changes are small from a logical point of view, they generate a fairly large patch due to impact of refactoring (sorry!). Most of the patch derives from the first two changes listed below.
> 
> The changes are summarized here:
> 
> #### 1. Generalize how we categorize references
> 
> The `Ref` class hierarchy models the various ways in which, at any point during the execution of a constructor or some other method/constructor that it invokes, there can be live references to the original object under construction lying around. We then look for places where one of these `Ref`'s might be passed to a subclass method. In other words, the analyzer keeps track of these references and watches what happens to them as the code executes so it can catch them trying to "escape".
> 
> Previously the `Ref` categories were:
> * `ThisRef` - The current instance of the (non-static) method or constructor being analyzed
> * `OuterRef` - The current outer instance of the (non-static) method or constructor being analyzed
> * `VarRef` - A local variable or method parameter currently in scope
> * `ExprRef` - An object reference sitting on top of the Java execution stack
> * `YieldRef` - The current switch expression's yield value(s)
> * `ReturnRef` - The current method's return value(s)
> 
> For each of those types, we further classified the "indirection" of the reference, i.e., whether the reference was direct (from the thing itself) or indirect (from something the thing referenced).
> 
> The problem with that hierarchy is that we could only track outer instance references that happened to be associated with the current instance. So we might know that `this` had an outer instance reference, but if we said `var x = this` we wouldn't know that `x` had an outer instance reference.
> 
> In other words, we should be treating "via an outer instance" as just another flavor of indirection along with "direct" and "indirect".
> 
> As a result, with this patch the `OuterRef` class goes away and a new `Indirection` enum has been created, with values `DIRECT`, `INDIRECT`, and `OUTER`.
> 
> #### 2. Track the types of all references
> 
> By keeping track of the actual type of...

Archie Cobbs has updated the pull request incrementally with one additional commit since the last revision:

  Synchronize TreeInfo.isExplicitThisReference() with PR #18088.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/16208/files
  - new: https://git.openjdk.org/jdk/pull/16208/files/e09e4229..304a92b6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16208&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16208&range=03-04

  Stats: 57 lines in 2 files changed: 35 ins; 21 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/16208.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16208/head:pull/16208

PR: https://git.openjdk.org/jdk/pull/16208