environ-selftests: Ignore -Wself-move warning

environ-selftests: Ignore -Wself-move warning

clang gives this warning:

 ..../gdb/unittests/environ-selftests.c:139:7: error: explicitly moving variable of type 'gdb_environ' to itself [-Werror,-Wself-move]
   env = std::move (env);
   ~~~ ^            ~~~

Ignoring the warning locally is the right thing to do, since it warns
about behavior we want to unit test, while an explicit self-move in
real code would likely be a mistake that we'd want to catch.

To avoid cluttering the code with preprocessor conditionals, this
commit adds the file common/diagnostics.h, in which we can put macros
used to control compiler diagnostics.

GCC enhancement request here:
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81159

gdb/ChangeLog:
2017-06-22  Pedro Alves  <palves@redhat.com>
	    Simon Marchi  <simon.marchi@ericsson.com>

	* unittests/environ-selftests.c (run_tests): Ignore -Wself-move
	warning.
	* common/diagnostics.h: New file.