Extend -remap-file=from;to to permit mapping from a non-existent

file. This is accomplished by introducing the notion of a "virtual"
file into the file manager, which provides a FileEntry* for a named
file whose size and modification time are known but which may not
exist on disk.

Added a cute little test that remaps both a .c file and a .h file it
includes to alternative files.

llvm-svn: 90329