RFR: JDK-8273914: Indy string concat changes order of operations [v4]

[Liam Miller-Cushon]

> This change makes string concatenation call `toString` on the arguments eagerly, to preserve the correct evaluation order of the arguments and the calls to `toString`.

Liam Miller-Cushon has updated the pull request incrementally with one additional commit since the last revision:

  Fix eager toString conversion for compound assignment

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5844/files
  - new: https://git.openjdk.java.net/jdk/pull/5844/files/bd4cb71e..53dddf6d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5844&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5844&range=02-03

  Stats: 23 lines in 2 files changed: 12 ins; 0 del; 11 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5844.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5844/head:pull/5844

PR: https://git.openjdk.java.net/jdk/pull/5844