We have an issue with `ELFDumper<ELFT>::getSymbolSectionName`:
1) It is used deeply for both LLVM/GNU styles and might return LLVM-style only
values to describe symbols: "Undefined", "Processor Specific", "Absolute", etc.
2) `getSymbolSectionName` is used by `getFullSymbolName` and these special values
might appear instead of symbol names in many places.
This occurs for unnamed section symbols currently.
This patch extracts the LLVM specific logic to `LLVMStyle<ELFT>::printSymbolSection`,
which seems to be the only place where we want to print the special values mentioned.
It also adds a meaningful new warning that is reported when we are unable to get
a section index for a section symbol.
Differential revision: https://reviews.llvm.org/D87764
3caa549f90