Added additional overloaded operators for APSInt to match the operators of

APInt.

While some operators were already specifically overloaded for APSInt, others
resulted in using the overloaded operator methods in APInt, which would result
in the signedness bit being lost.

Modified the APSInt(APInt&) constructor to be "explicit" and to take an
extra (optional) flag to indicate the signedness.  Making the ctor explicit
will catch any implicit conversations between APSInt -> APInt -> APSInt that
results in the signedness flag being lost.

llvm-svn: 46316