[ELF] .gdb_index: fix CuOff when a .debug_info section contains more than 1 DW_TAG_compile_unit

Summary:
Idx passed to readPubNamesAndTypes was an index into Chunks, not an
index into the CU list. This would be incorrect if some .debug_info
section contained more than 1 DW_TAG_compile_unit.

In real world, glibc Scrt1.o is a partial link of start.os abi-note.o init.o and contains 2 CUs in debug builds.
Without this patch, any application linking such Scrt1.o would have invalid .gdb_index
The issue could be demonstrated by:

    (gdb) py print(gdb.lookup_global_symbol('main'))
    None

Reviewers: espindola, ruiu

Reviewed By: ruiu

Subscribers: Higuoxing, grimar, dblaikie, emaste, aprantl, arichardson, JDevlieghere, arphaman, llvm-commits

Differential Revision: https://reviews.llvm.org/D54361

llvm-svn: 346747