[ELF] Rename NameTypeEntry to NameAttrEntry and its field "Type" to CuIndexAndAttrs

Summary:
NameTypeEntry::Type is a bit-packed value of CU index+attributes (https://sourceware.org/gdb//onlinedocs/gdb/Index-Section-Format.html), which is named cu_index_and_attrs in a local variable in gdb/dwarf2read.c:dw2_symtab_iter_next

The new name CuIndexAndAttrs is more meaningful.

Reviewers: ruiu, dblaikie, espindola

Reviewed By: dblaikie

Subscribers: emaste, aprantl, arichardson, JDevlieghere, arphaman, llvm-commits

Differential Revision: https://reviews.llvm.org/D54481

llvm-svn: 346794