Support: Add RedirectingFileSystem::create from simple list of redirections

Add an overload of `RedirectingFileSystem::create` that builds a
redirecting filesystem off of a simple vector of string pairs. This is
intended to be used to support `clang::arcmt::FileRemapper` and
`clang::PreprocessorOptions::RemappedFiles`.

Differential Revision: https://reviews.llvm.org/D91317