Move gdb/common/diagnostics.h to include/diagnostics.h

Move gdb/common/diagnostics.h to include/diagnostics.h so that it can
be used in binutils.

gdb/

	* ada-lex.l: Include "diagnostics.h" instead of
	"common/diagnostics.h".
	* unittests/environ-selftests.c: Likewise.
	* common/diagnostics.h: Moved to ../include.

include/

	* diagnostics.h: Moved from ../gdb/common/diagnostics.h.