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.

src/liballoc/lib.rs

@@ -69,6 +69,7 @@
 
 #![feature(no_std)]
 #![no_std]
+#![feature(allocator)]
 #![feature(lang_items, unsafe_destructor)]
 #![feature(box_syntax)]
 #![feature(optin_builtin_traits)]