swift234

FlaggedPointer should only compose with itself

<rdar://problem/19104060>

LLVM's PointerIntPair always picks the highest bits of the available low
order free bits but NumLowBitsAvailable is the number of *lowest* bits
available from the LSB. This could cause problems with overwriting when
putting a FlaggedPointer inside a PointerIntPair. Just don't allow it.

Add a static assert to make sure there are enough bits to support the
number of nested FlaggedPointers.

Add a helper constexpr min function which isn't available until C++14.

Swift SVN r23597