Don't raise an error for optimized out sub-fields.

Don't raise an error for optimized out sub-fields.

  http://sourceware.org/ml/gdb-patches/2013-07/msg00317.html

gdb/ChangeLog

        * value.c (value_fetch_lazy): Mark optimized out values as such
        rather than raising an error.

gdb/testsuite/ChangeLog

        * gdb.dwarf2/pieces-optimized-out.exp: Expect "<optimized out>"
        when printing an optimized out value.  Expect an error when using
        an optimized out value in an expression.