for review: fix for inlining of polymorphic procedures

[Thomas Charles CONWAY]

> 
> compiler/mercury_builtin.m
> compiler/code_util.m
> 	Add a new builtin, unsafe_type_cast/2, used by common.m
> 	to preserve type correctness.
> 	Make unsafe_promise_unique/2 a builtin, since it is basically
> 	the same as unsafe_type_cast/2.
> 

If this might occur in user code (as opposed to compiler generated
intermediate code), I guess you'd better to a Prolog version in
the library so that it works in Sicstus.

Thomas
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Thomas Conway               				      conway at cs.mu.oz.au
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