[ELF] Simplify adding default atoms

Now 'writer' creates an instance of `RuntimeFile` in the constructor, then
populates the file in the virtual function `addDefaultAtoms`, then pass
owning of this file to the caller of virtual function `createImplicitFiles`.

First, we do not need to keep an instance of `RuntimeFile` so long. It is
enough to create the file, right after that populate it and pass the owning.

Second, relationship between `createImplicitFiles` and `addDefaultAtoms`
is complicated. The `createImplicitFiles` might call `addDefaultAtoms`,
overridden version of `addDefaultAtoms` might call base class `addDefaultAtoms`,
and overridden version of `createImplicitFiles` might call base class
`createImplicitFiles` as well as `addDefaultAtoms`.

The patch solves both problems above. It creates and populates runtime files
right in the createImplicitFiles(), removes `addDefaultAtoms` at all and
does not keep references to runtime files in class fields.

llvm-svn: 234347