Improve bootstrapping of unary concatenations

Expressions like "" + foo ends up as a unary concatenation: a makeConcatWithConstants taking only foo as an argument along with a recipe of "\u0001"

For all primitive types, this can be reduced to handles to the equivalent String.valueOf. For reference types we need a specialized variant of the StringConcatHelper.stringOf(Object) that explicitly creates a new String, to comply with JLS 15.18.1

This can improve startup and bootstrap overheads of such simple expressions