[LibTooling] Fix unneeded use of unique_ptr where shared_ptr is expected.

Summary: This fixes a few places in the Stencil implementation where a unique_ptr is created at a callsite that expects shared_ptr. Since the former implicitly converts to the latter, the code compiles and runs correctly as is.  But, there's no reason to involve unique_ptr -- the current code was leftover from a previous version in which unique_ptr was the expected type.

Reviewers: sbenza

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61005

llvm-svn: 359468