gdb_vecs.h: Avoid self move assign

gdb_vecs.h: Avoid self move assign

While working on another patch I ran into an issue with
unordered_remove (in gdb_vecs.h), where removing the last item of the
vector can cause a self move assign.

When compiling the C++ standard library in debug mode (with
-D_GLIBCXX_DEBUG=1) this causes an error to trigger.

I've fixed the issue in this patch and provided a unit test.

The provided unit test includes an assignment operator which checks
for self move assign, this removes the need to compile with
-D_GLIBCXX_DEBUG=1 in order to spot the bug.  If you're keen to see
the error reported from the C++ standard library then remove operator=
from the unit test and recompile GDB with -D_GLIBCXX_DEBUG=1.

gdb/ChangeLog:

	* Makefile.in (SUBDIR_UNITTESTS_SRCS): Add new file to the list.
	* unittests/vec-utils-selftests.c: New file.
	* gdbsupport/gdb_vecs.h (unordered_remove): Avoid self move assign.

Change-Id: I80247b20cd5212038117db7412865f5e6a9257cd