[clang-tidy] Move formatDereference to FixitHintUtils

I'd like to use RedundantStringCStrCheck's formatDereference function
from the up-coming modernize-use-std-print check. Let's move it to
FixItHintUtils so that the implementation can be shared.

Reviewed By: PiotrZSL

Differential Revision: https://reviews.llvm.org/D150602