Change setDiagnosticsOutputFile to take a unique_ptr from a raw pointer (NFC)

Summary:
This makes it explicit that ownership is taken. Also replace all `new`
with make_unique<> at call sites.

Reviewers: anemet

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D26884

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@287449 91177308-0d34-0410-b5e6-96231b3b80d8