gdb/dwarf: avoid cutu_reader moves

gdb/dwarf: avoid cutu_reader moves

In process_psymtab_comp_unit and ensure_cu_exists, we create a temporary
cutu_reader on the stack, then move it to a heap allocated cutu_reader
once we confirmed the unit is not dummy.  I think it's unnecessary to
create a temporary cutu_reader.  The only downside of not doing so is that if it
ends up that the CU is dummy, we made an allocation/deallocation for
nothing.  Dummy CUs are a rare thing, it shouldn't change anything.

This allows removing the cutu_reader move constructor.

Change-Id: I44742d471c495055ee46db41c0e7bdfbd2d5c0b7
Approved-By: Tom Tromey <tom@tromey.com>