RFR: 8288425: Footprint regression due MH creation when initializing StringConcatFactory

[Claes Redestad]

Avoid doing MH creation when initializing `StringConcatFactory` by lazily initializing to a `@Stable` field instead.

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Commit messages:
 - 8288425: Footprint regression due MH creation when initializing StringConcatFactory

Changes: https://git.openjdk.org/jdk/pull/9154/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=9154&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288425
  Stats: 11 lines in 1 file changed: 7 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/9154.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9154/head:pull/9154

PR: https://git.openjdk.org/jdk/pull/9154