gdb: allocate subfile with new

gdb: allocate subfile with new

Allocate struct subfile with new, initialize its fields instead of
memset-ing it to 0.  Use a unique_ptr for the window after a subfile has
been allocated but before it is linked in the buildsym_compunit's list
of subfile (and therefore owned by the buildsym_compunit.

I can't test the change in xcoffread.c, it's best-effort.  I couldn't
find where subfiles are freed in that file, I assume they were
intentionally (or not) leaked.

Change-Id: Ib3b6877de31b7e65bc466682f08dbf5840225f24