PR5207: Change APInt methods trunc(), sext(), zext(), sextOrTrunc() and

PR5207: Change APInt methods trunc(), sext(), zext(), sextOrTrunc() and
zextOrTrunc(), and APSInt methods extend(), extOrTrunc() and new method
trunc(), to be const and to return a new value instead of modifying the
object in place.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@121121 91177308-0d34-0410-b5e6-96231b3b80d8