[clang-tidy] Handle data() in readability-redundant-string-cstr

Summary:
std::string::data() and std::string::c_str() are equivalent.
Enhance the readability-redundant-string-cstr check to also handle
calls to data().

Reviewers: etienneb, alexfh, aaron.ballman

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D26279

llvm-svn: 285901