[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.