Handle correctly containers that are data members in modernize-loop-convert.

Summary:
I recently found that the variable naming wasn't working as expected with containers that are data members. The new index always received the name "Elem" (or equivalent) regardless of the container's name.
The check was assuming that the container's declaration was a VarDecl, which cannot be converted to a FieldDecl (a data member), and then it could never retrieve its name.

This also fixes some cases where the check failed to find the container at all (so it didn't do any fix) because of the same reason.

Reviewers: klimek

Subscribers: cfe-commits, alexfh

Differential Revision: http://reviews.llvm.org/D14289

llvm-svn: 251943