CodeGen: Have 'this'-returning constructors and destructors to take advantage of the new backend 'returned' attribute.

The backend will now use the generic 'returned' attribute to form tail calls where possible, as well as avoid save-restores of 'this' in some cases (specifically the cases that matter for the ARM C++ ABI).

This patch also reverts a prior front-end only partial implementation of these optimizations, since it's no longer required.
Add a version of unique_file that return just the file name.