[AST] Use PrintingPolicy for format string diagnosis

Summary:
This is a small improvement for OpenCL diagnostics, but is also useful
for our CHERI fork, as our __capability qualifier is suppressed from
diagnostics when all pointers are capabilities, only being used when pointers
need to be explicitly opted-in to being capabilities.

Reviewers: rsmith, Anastasia, aaron.ballman

Reviewed By: Anastasia, aaron.ballman

Subscribers: aaron.ballman, arichardson, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78777