Efficient hidden class-based string concatenation strategy

The current strategy for bootstrapping JEP 280 String concat expressions uses either an optimized MH expression tree, or, due to scalability issues with that implementation, a per call-site StringBuilder-based class generation scheme.

This RFE seeks to unify these into a single strategy that uses per-shape class generation emitting code semantically similar to the optimized MH expression tree. Using hidden classes installed into java.lang with access to the StringConcatHelper set of utility methods this can achieve similar throughput performance, while reducing the number of generated classes at high arities, reducing startup overheads, and improving warmup characteristics.

This RFE was initially prompted by a suggestion of [~redestad] in the discussion of PR #20253 (https://github.com/openjdk/jdk/pull/20253#issuecomment-2240412866), and then evolved after collaborating on this issue into this full re-implementation.