[ELF] Make LinkerScript::assignAddresses iterative

PR42990. For `SECTIONS { b = a; . = 0xff00 + (a >> 8); a = .; }`,
we currently set st_value(a)=0xff00 while st_value(b)=0xffff.

The following call tree demonstrates the problem:

```
link<ELF64LE>(Args);
  Script->declareSymbols(); // insert a and b as absolute Defined
  Writer<ELFT>().run();
    Script->processSectionCommands();
      addSymbol(cmd);       // a and b are re-inserted. LinkerScript::getSymbolValue
                            // is lazily called by subsequent evaluation
    finalizeSections();
      forEachRelSec(scanRelocations<ELFT>);
        processRelocAux     // another problem PR42506, not affected by this patch
      finalizeAddressDependentContent(); // loop executed once
        script->assignAddresses(); // a = 0, b = 0xff00
    script->assignAddresses(); // a = 0xff00, _end = 0xffff
```

We need another assignAddresses() to finalize the value of `a`.

This patch

1) modifies assignAddress() to track the original section/value of each
  symbol and return a symbol whose section/value has changed.
2) moves the post-finalizeSections assignAddress() inside the loop
  of finalizeAddressDependentContent() and makes it iterative.
  Symbol assignment may not converge so we make a few attempts before
  bailing out.

Note, assignAddresses() must be called at least twice. The penultimate
call finalized section addresses while the last finalized symbol values.
It is somewhat obscure and there was no comment.
linkerscript/addr-zero.test tests this.

Reviewed By: ruiu

Differential Revision: https://reviews.llvm.org/D66279

llvm-svn: 369889