[RFA] Use std::string in reread_symbols

[Simon Marchi]

On 2018-05-19 11:17 AM, Tom Tromey wrote:
> This removes a cleanup from reread_symbols by using std::string.  This
> fixes a memory leak, because this cleanup is ordinarily discarded, not
> run.
> 
> Tested by the buildbot.
LGTM, thanks.

Simon