Introduce generateCodeForArgument() in CodeFlow

[sbrannen]

Overview

SpelNodeImpl currently defines a generateCodeForArgument() utility method which "asks an argument to generate its bytecode and then follow it up with any boxing/unboxing/checkcasting to ensure it matches the expected parameter descriptor."

This utility method is currently protected and cannot be used outside the org.springframework.expression.spel.ast; however, there are times when it would be beneficial to be able to invoke this method from types within the org.springframework.expression.spel package that deal with compilation.

This is a prerequisite for the forthcoming CompilableIndexAccessor.

Related Issues

• Introduce CompilableIndexAccessor SPI in SpEL #32613
• Make SpelNode compilation aware #32707