Support FirStringConcatenationCall in FirCompileTimeConstantEvaluator.
This allows string templates ("foo${bar}") to be evaluated as constants,
assuming the interpolated expressions are themselves constant.
In addition, fixes some handling bugs with KtConstantEvaluationMode,
where some expressions that are not valid in a `const val` declaration
were being supported for `CONSTANT_EXPRESSION_EVALUATION`, including
non-static final Java fields in FIR, and composite expressions of
non-const properties in FE1.0.
Justin Paupore