[llvm] Expose DWARFDebugLine::LineTable::getFileNameEntry

Summary:
This is useful for scenarios where Prologue was directly used and DWARF
5 awareness is required. The current alternative would be to either
duplicate the logic in getFileNameEntry, or to use getFileNameByIndex.
The latter isn't quite an in-place replacement - it performs some
processing, and it produces a string instead of a StringRef, meaning
the caller needs to handle its lifetime.

Reviewers: tamur, dblaikie, JDevlieghere

Reviewed By: tamur, JDevlieghere

Subscribers: aprantl, llvm-commits

Tags: #llvm, #debug-info

Differential Revision: https://reviews.llvm.org/D63228

llvm-svn: 363210