IntersectionTypeBinding18.erasure() may return a non-reifiable result

[stephan-herrmann]

Preparation: remove now-dead code.

Relates to #3646

[stephan-herrmann]

Reviving what fell between the cracks.

• dead code previously removed no longer exists in master
• much simpler implementation of ITB18.erasure()
• redirect constantPoolName() through the improved erasure() for consistency
• adjusted one code location that worked (i.e., not emitting an unwanted checkcast) only because erasure() returned the ITB18 unchanged (as pointed out be intermediate regressions)

[stephan-herrmann]

@srikanth-sankaran do you agree on putting this on our list for 4.40 M1? I'm not aware of any observable defect we are fixing here (while of course internally the need for change is obvious).

[srikanth-sankaran]

@srikanth-sankaran do you agree on putting this on our list for 4.40 M1? I'm not aware of any observable defect we are fixing here (while of course internally the need for change is obvious).

Sounds good.