VFS overlay file allow using relative paths in root->name paths (root directory of the virtual file) and external-contents (the actual files). While the 'external-contents' could be configured to relative to directory of the YAML file (change https://reviews.llvm.org/D17457), the root paths could only be relative to the current working directory.
This patch add the "root-relative" option to the YAML file format. When this option is set to "yaml-dir" the root->name will be prepend by the YAML file directory. This option is helpful when compiling on case sensitive file systems when cross compiling to Windows as we can create a vfsoverlay YAML file for the Windows libraries without using absolute paths. Related change: https://reviews.llvm.org/D125800