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.