[compiler-rt] Make the MSAN wmemset intercepter call wmemset instead of memset. Fixes PR 21579

Summary:
Exactly what the title says. I've tested this change against the libc++ test failures and it solves all of them. The check-msan rule also still passes.
I'm not sure why it called memset originally.

I can add tests if requested but currently there are no tests involving wide chars and they are a c++11 features.

Reviewers: kcc, eugenis

Reviewed By: eugenis

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6352