[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@283766 91177308-0d34-0410-b5e6-96231b3b80d8