swift234

Dont' trivial bitcast between differently sized structs.

Really we want to only cast between equally sized types. SIL does not really
have a concept of types of the same size so this solution is a workaround
(temporary) for acutal code in the standard library which casts a pointer of an
UTF16 type to a pointer of an UTF8 type.

rdar://18118602

Swift SVN r21470