[ptr-traits] Add an #include to provide the complete type for Section

which is the pointee in a pointer keyed DenseMap.

Pointer keys in a DenseMap have their low bits used by the map
implementation and in order to do this safely we need to know the
alignment of the pointer. We can only do this if we have the complete
type of the pointee. This is part of larger cleanup across LLVM and
subprojects to allow us to start enforcing this strictly.

llvm-svn: 256616