[Reproducers] Make reproducer relocatable

Before this patch, reproducers weren't relocatable. The reproducer
contained hard coded paths in the VFS mapping, as well in the yaml file
listing the different input files for the command interpreter. This
patch changes that:

 - Use relative paths for the DataCollector.
 - Use an overlay prefix for the FileCollector.

Differential revision: https://reviews.llvm.org/D63467

llvm-svn: 363697