irgen, driver: modify Compiler.Compile to take a FileSet and Files

This change allows clients to generate IR using "files" received from locations
other than the file system. The regular file parser is moved to a new library,
"driver", which is intended to eventually contain much of the logic from
the existing driver.

Differential Revision: http://reviews.llvm.org/D6794

llvm-svn: 225026