[sanitizer] Add strnlen to the common interceptors

Summary:
Adds strnlen to the common interceptors, under the existing flag
intercept_strlen.

Removes the now-duplicate strnlen interceptor from asan and msan.
This adds strnlen to tsan, which previously did not intercept it.

Adds a new test of strnlen to the sanitizer_common test cases.

Reviewers: samsonov

Subscribers: zhaoqin, llvm-commits, kcc

Differential Revision: http://reviews.llvm.org/D18397

llvm-svn: 264195