[lld] For --defsym from=to, retain `to` only if `from` is also retained, rather than retain `to` unconditionally

[MaskRay]

Originally reported at https://lists.llvm.org/pipermail/llvm-dev/2019-December/137403.html

After https://reviews.llvm.org/D34195 , we retain to unconditionally for a symbol assignment --defsym from=to (The expectation is that there may be some code referencing to.) This is overly conservative. If from is not retained, we don't actually have to retain to.

% cat a.s
.globl _start, foo, bar
.text; _start: movabs $d, %rax
.section .text_foo,"ax"; foo: ret
.section .text_bar,"ax"; bar: nop
% as a.s -o a.o
% ld.lld a.o --defsym c=foo --defsym d=1 --gc-sections -o a
# .text_foo is retained, though `c` is not actually used.

It is difficult to drop the following rule in MarkLive.cpp:

  for (StringRef s : script->referencedSymbols)
    markSymbol(symtab->find(s));

The issue can be demonstrated by the following call tree:

LinkerDriver::link
  markLive
    ...
    resolveReloc
      // Defined::section is nullptr for `d` because the assignment d=foo hasn't been evaluated yet.
  writeResult
    Writer<ELFT>::run
      Writer<ELFT>::finalizeSections
        LinkerScript::processSymbolAssignments
          // Symbol section+offset are evaluated here.

The placement of processSymbolAssignments is a bit flexible. It can be reorder to just after processSectionCommands. Adding another processSymbolAssignments can solve the problem, but we need to be very careful adding more passes. There are lots of intricate dependencies. We need to find a balance between benefits/costs.

This enhancement is probably of low priority. It may not worth a fix on its own, but if reordering passes can address other problems, it would be nice to improve --defsym as well.

I have also studies GNU ld's behavior. It turns out that it is not reliable in some cases:

ld.bfd a.o --defsym 'd=foo' --gc-sections -o a => GNU ld retains .text_foo
ld.bfd a.o --defsym 'd=foo+3' --gc-sections -o a => GNU ld drops .text_foo
ld.bfd a.o --defsym 'd=bar-bar+foo' --gc-sections -o a => GNU ld drops .text_foo