RFR (S) 8149459: StringConcatFactory should be synced up with LambdaMetafactory

[Aleksey Shipilev]

Hi,

I would like to make StringConcatFactory more coherent with existing
LambdaMetafactory:
  https://bugs.openjdk.java.net/browse/JDK-8149459
  http://cr.openjdk.java.net/~shade/8149459/webrev.01/

Testing: jdk/test/java/String/concat jtreg, JPRT

Cheers,
-Aleksey