[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

llvm-svn: 283271