Support resetting STATISTIC() values using llvm::ResetStatistics()

Summary:
Most of the time, compiler statistics can be obtained using a process that
performs a single compilation and terminates such as llc. However, this isn't
always the case. JITs for example, perform multiple compilations over their
lifetime and STATISTIC() will record cumulative values across all of them.

Provide tools like this with the facilities needed to measure individual
compilations by allowing them to reset the STATISTIC() values back to zero using
llvm::ResetStatistics(). It's still the tools responsibility to ensure that they
perform compilations in such a way that the results are meaningful to their
intended use.

Reviewers: qcolombet, rtereshin, bogner, aditya_nandakumar

Reviewed By: bogner

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D44181

llvm-svn: 326981