PointerLikeTypeTraits: Standardize NumLowBitsAvailable on static constexpr...

PointerLikeTypeTraits: Standardize NumLowBitsAvailable on static constexpr rather than anonymous enum

This is (more?) usable by GDB pretty printers and seems nicer to write.

There's one tricky caveat that in C++14 (LLVM's codebase today) the
static constexpr member declaration is not a definition - so odr use of
this constant requires an out of line definition, which won't be
provided (that'd make all these trait classes more annoyidng/expensive
to maintain). But the use of this constant in the library implementation
is/should always be in a non-odr context - only two unit tests needed to
be touched to cope with this/avoid odr using these constants.

Based on/expanded from D72590 by Christian Sigg.