RFR: JDK-8273914: Indy string concat changes order of operations.

[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`.

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Commit messages:
 - JDK-8273914: Indy string concat changes order of operations.

Changes: https://git.openjdk.java.net/jdk/pull/5844/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5844&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273914
  Stats: 383 lines in 7 files changed: 345 ins; 0 del; 38 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