[Sanitizer] Add interceptor for wctomb
This is required to avoid msan false positives for code using this
function (although generally one should avoid using this function in
favor of wcrtomb).
Reviewers: eugenis, EricWF, vitalybuka
Subscribers: srhines, kubamracek, fedor.sergeev, delcypher, llvm-commits, #sanitizers
Tags: #llvm, #sanitizers
Differential Revision: https://reviews.llvm.org/D59548