ADT: Add int64_t interoperability to APSInt

Add some API to `APSInt` to make it easier to compare with `int64_t`.

  - `APSInt::compareValues(APSInt, APSInt)` returns 1, -1 or 0 for
    greater, lesser, or equal, doing the right thing for mismatched
    "has-sign" and bitwidths.  This is just like `isSameValue()` (and is
    now the implementation of it).
  - `APSInt::get(int64_t)` gets a signed `APSInt`.
  - `operator<(int64_t)`, etc., are implemented trivially via `get()`
    and `compareValues()`.
  - Also added `APSInt::getUnsigned(uint64_t)` to make it easier to test
    `compareValues()`.

llvm-svn: 228239