[llvm] Expose DWARFDebugLine::LineTable::getFileNameEntry

[mtrofin]

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.

llvm/include/llvm/DebugInfo/DWARF/DWARFDebugLine.h

struct LineTable {
 
     /// Parse prologue and all rows.
     Error parse(
         DWARFDataExtractor &DebugLineData, uint32_t *OffsetPtr,
         const DWARFContext &Ctx, const DWARFUnit *U,
         std::function<void(Error)> RecoverableErrorCallback,
         raw_ostream *OS = nullptr);
 
+    /// Get DWARF-version aware access to the file name entry at the provided
+    /// index.
+    const llvm::DWARFDebugLine::FileNameEntry &
+        getFileNameEntry(uint64_t Index) const;
+
     using RowVector = std::vector<Row>;
     using RowIter = RowVector::const_iterator;
     using SequenceVector = std::vector<Sequence>;
     using SequenceIter = SequenceVector::const_iterator;
 
     struct Prologue Prologue;
     RowVector Rows;
     SequenceVector Sequences;
 
   private:
-    const llvm::DWARFDebugLine::FileNameEntry &
-    getFileNameEntry(uint64_t Index) const;
     uint32_t findRowInSeq(const DWARFDebugLine::Sequence &Seq,
                           object::SectionedAddress Address) const;
     Optional<StringRef>
     getSourceByIndex(uint64_t FileIndex,
                      DILineInfoSpecifier::FileLineInfoKind Kind) const;
 
     uint32_t lookupAddressImpl(object::SectionedAddress Address) const;