[lworld] RFR: 8263900: [lworld] Make .default a separate node type in the parser. [v2]

[Jesper Steen Møller]

> To prepare for making .default a poly expression and to implement further logic, .default-expressions should be split off as a separate AST node.

Jesper Steen Møller has updated the pull request incrementally with one additional commit since the last revision:

  Corrected as per Srikanth's review:
  
  (1) Renamed DefaultExpression etc. to DefaultValue (and DEFAULT_EXPRESSION to DEFAULT_VALUE)
  (2) and (3) getClazz is now getType and returns JCExpression
  (4) Relevant visitor added to SourceComputer
  (5), (6), and (8) cleaned up
  
  (7) has not been addressed, as it appears to be an unrelated bug.

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

Changes:
  - all: https://git.openjdk.java.net/valhalla/pull/370/files
  - new: https://git.openjdk.java.net/valhalla/pull/370/files/dd883c15..adc1372f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=valhalla&pr=370&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=valhalla&pr=370&range=00-01

  Stats: 47 lines in 17 files changed: 7 ins; 3 del; 37 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/370.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/370/head:pull/370

PR: https://git.openjdk.java.net/valhalla/pull/370