[DebugInfo] Remove Dwarf5AccelTableWriter::Header::UnitLength. NFC.

[ikudrin]

Summary

The member is not in use; the unit length for the table is emitted as a difference between two labels. Moreover, the type of the member might be misleading, because for DWARF64 the field should be 64 bit long.

[dblaikie]

Sounds good

[jhenderson]

Makes sense to me too.

llvm/lib/CodeGen/AsmPrinter/AccelTable.cpp

 ///
 /// The class is templated in its data type. This allows us to emit both dyamic
 /// and static data entries. A callback abstract the logic to provide a CU
 /// index for a given entry, which is different per data type, but identical
 /// for every entry in the same table.
 template <typename DataT>
 class Dwarf5AccelTableWriter : public AccelTableWriter {
   struct Header {
-    uint32_t UnitLength = 0;
     uint16_t Version = 5;
     uint16_t Padding = 0;
     uint32_t CompUnitCount;
     uint32_t LocalTypeUnitCount = 0;
     uint32_t ForeignTypeUnitCount = 0;
     uint32_t BucketCount;
     uint32_t NameCount;
     uint32_t AbbrevTableSize = 0;