[ELF] - Simplify implementation of constant pool when building .gdb_index

https://sourceware.org/gdb/onlinedocs/gdb/Index-Section-Format.html says:
"A CU vector in the constant pool is a sequence of offset_type values. 
The first value is the number of CU indices in the vector. 
Each subsequent value is the index and symbol attributes of a CU in the CU list."

Previously we keeped 2 values until the end, what was useless. 
Initially was a part of D32647, though it is possible to split out. 
Patch do that.

Differential revision: https://reviews.llvm.org/D33551

llvm-svn: 303973