Make File non-const in the resolver.

File objects are not really const in the resolver. We set ordinals to
them and call beforeLink hooks. Also, File's member functions marked
as const are not really const. ArchiveFile never returns the same
member file twice, so it remembers files returned before. find() has
side effects.

In order to deal with the inconsistencies, we sprinkled const_casts
and marked member varaibles as mutable.

This patch removes const from there to reflect the reality.

llvm-svn: 231212