[Sanitizer] Add interceptor for wctomb

Summary:
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

llvm-svn: 357240