RFR: JDK-8273914: Indy string concat changes order of operations [v5]

[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:

  Rename WellKnownTypes to WellKnownTypeSignatures
  
  and add more functional test coverage for string concat of well-known
  types, covering all strategies.

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5844/files
  - new: https://git.openjdk.java.net/jdk/pull/5844/files/53dddf6d..d23a324d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5844&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5844&range=03-04

  Stats: 78 lines in 2 files changed: 3 ins; 41 del; 34 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