[CUDA] Destroy deferred diagnostics before destroying the ASTContext's PartialDiagnostic allocator.

Summary:
This will let us (in a separate patch) allocate deferred diagnostics in
the ASTContext's PartialDiagnostic arena.

Reviewers: rnk

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D25260

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@283271 91177308-0d34-0410-b5e6-96231b3b80d8