Refactor declarations of ASTContext allocate functions into its own header.

Forward declarations of the allocate functions combine with the forward
declaration of the ASTContext class is enough information for some headers
without pulling in ASTContext.h in its entirety.  Pull the existing
declarations from AttrIterator.h into a new header.  Also place the default
alignment size into this header.  Previously, new had its default in
AttrIterator.h while new[] had its default in ASTContext.h.  Add new header
includes where it is needed.  Specifically to ASTVector.h to make it a
standalone header, unlike previously which it was standalone as long as
none of its functions were called.

llvm-svn: 350792