Add an "allocator" attribute to mark functions as allocators

Add an "allocator" attribute to mark functions as allocators

When this attribute is applied to a function, its return value gets the
noalias attribute, which is how you tell LLVM that the function returns
a "new" pointer that doesn't alias anything accessible to the caller,
i.e. it acts like a memory allocator.

Plain malloc doesn't need this attribute because LLVM already knows
about malloc and adds the attribute itself.