Revert "cpp11-migrate: Write header replacements to disk"

This reverts commit 187428. It broke the windows bots.
http://bb.pgr.jp/builders/ninja-clang-i686-msc17-R/builds/3450

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@187447 91177308-0d34-0410-b5e6-96231b3b80d8