Substantial cleanups:

 * Add header comment
 * Remove extraneous #includes
 * Move the FileType enum into the GCC class
 * The GCC class is not virtual.
 * Move all of the "constructor" functions into the classes themselves
 * Stop using cl::list as arguments, use std::vector instead (which cl::list
   derives from)
 * Improve comments

llvm-svn: 9121