Revert "D11003: Tolerate DWARF compile unit without filename."

Summary:
This code is modifying a support file list after it has been created.
This makes it hard to share the file list between type units and
compile units in DWARF. It's not a total showstopper, but supporting
this while also sharing the lists would make things more complicated.

Given that this was added to support a project which never fully
materialised, and that even back then there were some concerns about the
correctness of this approach (according to D11003#200772 the compile
unit name is not guaranteed to be the first one in the support file
list), I think we should just delete this workaround.

Reviewers: clayborg, tberghammer, dsrbecky

Subscribers: aprantl, lldb-commits

Differential Revision: https://reviews.llvm.org/D62517

llvm-svn: 361948