`String.+=` function to `String.append` function, and use a new
semantics attribute for String.+=.
Teach the constant evaluator about `String.Append` instead of `String.+=`.
unknown symbolic values by renaming some diagnostics and
creating new unknown reasons for each type of failure that
can happen during constant evaluation.
(also referred to as flow-sensitive mode) so that the evaluator
can be used by clients to constant evaluate instructions in a
SILFunction body one by one following the flow of control.
