[Support] Replace function with function_ref in writeFileAtomically. NFC

Summary:
The latter is slightly more efficient and communicates the intent of the
API: writeFileAtomically does not own or copy the callback, it merely
calls it at some point.

Reviewers: jkorous

Reviewed By: jkorous

Subscribers: hiraditya, dexonsmith, jfb, llvm-commits, cfe-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67584

llvm-svn: 372201