[Sema] Use unique_ptr instead of raw pointers in the late-parsed templates map.

[Sema] Use unique_ptr instead of raw pointers in the late-parsed templates map.

Summary:
This is possible now that MapVector supports move-only values.

Depends on D25404.

Reviewers: timshen

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D25405

llvm-svn: 283766