[VFS] Let the user decide if they want path normalization.

This is a more principled version of what I did earlier. Path
normalization is generally a good thing, but may break users in strange
environments, e. g. using lots of symlinks. Let the user choose and
default it to on.

This also changes adding a duplicated file into returning an error if
the file contents are different instead of an assertion failure.

Differential Revision: http://reviews.llvm.org/D13658

llvm-svn: 250060