RFR: 8317336: Assertion error thrown during 'this' escape analysis [v3]

[Archie Cobbs]

> The 'this' escape analyzer was assuming that lambas could always be treated like blocks, in the sense that we can assume a block can never end in a plain expression. This assumption is not true for lambda's of the form `() -> foo` which do exactly that. In the case that `foo` was `this`, an assertion failure would ensue.
> 
> This patch explicitly clears any leftover `ExprRef`'s after evaluating a lambda body; this restores the asserted invariant when the lambda body is an expression.
> 
> This also fixes another variant of the same bug caused by certain enhanced `for()` loops. In this case, the leftover `ExprRef` was from evaluating the loop expression. This was just a side-effect of the analyzer failing to override `TreeScanner` to actually assign the loop expression to the loop variable.

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

  Replace 8ad1247a with more correct fix.
  
  The mistaken assumption was that a lambda body is always a JCBlock.
  It can also be a JCExpression, in which case it could result in an
  ExprRef 'this' reference.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/16001/files
  - new: https://git.openjdk.org/jdk/pull/16001/files/03cda07e..457bcca6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16001&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16001&range=01-02

  Stats: 5 lines in 1 file changed: 3 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/16001.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16001/head:pull/16001

PR: https://git.openjdk.org/jdk/pull/16001