give PATypeHolder an explicit copy ctor which initializes the type pointer,

and make PATypeHolder work with null pointers.

The implicitly generated one didn't work on numerous levels, but was still
accepted, allowing all sorts of bugs with default constructed pa type holders.

Previously, they "sort of" worked if they were default constructed and then
destructed.  Now they really work, and you can even default construct one,
then assign to it, amazing.

llvm-svn: 107195