A clang-tidy check for std:accumulate.

Summary:
For folds (e.g. std::accumulate), check matches between the provided init value and the range's value_type. A typical error is "std::accumulate(begin, end, 0);", where begin and end have float value_type. See the documentation for more examples.

For now we check std::accumulate, std::reduce and std::inner_product.

Reviewers: hokein, alexfh

Subscribers: Prazek, aaron.ballman, cfe-commits, courbet

Patch by Clément Courbet!

Differential Revision: http://reviews.llvm.org/D18442

llvm-svn: 267542