RFR 8187237: Need to define the behaviour for 0 and 1 argument method type in StringConcatFactory.makeConcat

[Paul Sandoz]

8186737: Lookup argument for StringConcatFactory.makeConcat & 
makeConcatWithConstants cannot have privileges less than PRIVATE

Please review these minor specification updates to the StringConcat bootstrap 
methods to clarify behaviour:

  
http://cr.openjdk.java.net/~psandoz/jdk10/JDK-8186737-8187237-string-concat-bsm-spec-updates/webrev/

(I also updated the lambda bootstrap methods in the context of 8186737)

Thanks,
Paul.