[llvm-readobj/elf] - Stop reporting invalid extended indexes in warnings for...

[llvm-readobj/elf] - Stop reporting invalid extended indexes in warnings for unnamed section symbols.

We have an issue with `getFullSymbolName`: it assumes that the symbol passed is
always in the `.symtab`, what is wrong. We might calculate and report a wrong index currently.
I've added a test case revealing that.

This patch adds the "symbol index" argument to `getFullSymbolName` signature,
what fixes the issue.

Differential revision: https://reviews.llvm.org/D87899