[ELF] - Partial support of --gdb-index command line option (Part 3).

Patch continues work started in D24706 and D25821.

in this patch symbol table and constant pool areas were
added to .gdb_index section output.

This one finishes the implementation of --gdb-index functionality in LLD.

Differential revision: https://reviews.llvm.org/D26283

llvm-svn: 289810