[ADT] Update PointerIntPair to handle pointer types with more than 31 bits free.

[ADT] Update PointerIntPair to handle pointer types with more than 31 bits free.

Previously, the assertions in PointerIntPair would try to calculate the value
(1 << NumLowBitsAvailable); the inferred type here is 'int', so if there were
more than 31 bits available we'd get a shift overflow.

Also, add a rudimentary unit test file for PointerIntPair.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203273 91177308-0d34-0410-b5e6-96231b3b80d8