clang-tools-extra/test/cpp11-migrate/HeaderReplacements/main.cpp: Use FileCheck instead of grep(1).

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@188347 91177308-0d34-0410-b5e6-96231b3b80d8
clang-tools-extra/test/cpp11-migrate/HeaderReplacements/main.cpp: Use "%/t" instead of "%t" on sed(1) to avoid \'s expanded to control chars.

The feature, "%/t", has been introduced in llvm/lit since r188348.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@188349 91177308-0d34-0410-b5e6-96231b3b80d8