Statistic: Only print statistics on exit for -stats

Previously enabling the statistics with EnableStatistics() would lead to
them getting printed to stderr/-info-output-file on exit. However
frontends may want a way to enable statistics and do the printing on
their own instead of the forced printing on exit.

This changes the code so that only the -stats option enables printing on
exit, EnableStatistics() only enables the tracking but requires invoking
one of the PrintStatistics() variants.

Differential Revision: https://reviews.llvm.org/D24819

llvm-svn: 282425