clang-tools-extra/test/cpp11-migrate/HeaderReplacements/main.cpp: Use FileCheck instead of grep(1).
Lit: Introduce "%/[STpst] into parseIntegratedTestScript(), to normalize substitutions.
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.