[SLC] Convert some strndup calls to strdup calls

Summary:
Motivation:
- If we can fold it to strdup, we should (strndup does more things than strdup).
- Annotation mechanism. (Works for strdup well).

strdup and strndup are part of C 20 (currently posix fns), so we should optimize them.

Reviewers: efriedma, jdoerfert

Reviewed By: jdoerfert

Subscribers: lebedev.ri, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67679

llvm-svn: 372636