[asan] Rename tree-asan.[ch] to asan.[ch]

[Diego Novillo]

As discussed earlier, this patch renames tree-asan.[ch] to asan.[ch].

Tested on x86_64.  Committed to branch.


Diego.
    
        * asan.c: Rename from tree-asan.c.
        Update all users.
        * asan.h: Rename from tree-asan.h
        Update all users.