[clang-tidy] new cppcoreguidelines-narrowing-conversions check.

Summary:
Checks for narrowing conversions, e.g.

int i = 0;
i += 0.1;

This has what some might consider false positives for:
i += ceil(d);

Reviewers: alexfh, hokein

Subscribers: srhines, nemanjai, mgorny, JDevlieghere, xazax.hun, kbarton

Differential Revision: https://reviews.llvm.org/D38455

llvm-svn: 333066