[ELF] Simplify handling of exportDynamic and isPreemptible

In Writer::includeInDynSym(), exportDynamic is used by a Defined with
protected or default visibility, to record whether it is required to be
exported into .dynsym. It is set when any of the following conditions
hold:

1) There is an interposable symbol from a DSO (Undefined or SharedSymbol with default visibility)
2) If -shared or --export-dynamic is specified, any symbol in an object file/bitcode sets this property, unless suppressed by canBeOmittedFromSymbolTable().
3) --dynamic-list when producing an executable

4) protected symbol from a DSO preempted by copy relocation/canonical PLT when
  --ignore-{data,function}-address-equality is specified
5) ifunc is exported when -z ifunc-noplt is specified

Bullet points 4) and 5) are irrelevant in this patch.

Bullet 3) does not play well with 1) and 2). When -shared is specified,
exportDynamic of most symbols is true. This makes it incapable to record
--dynamic-list marked symbols. We thus have obscure:

    if (!config->shared)
      b->exportDynamic = true;
    else if (b->includeInDynsym())
      b->isPreemptible = true;

This patch adds another bit `Symbol::inDynamicList` to record
3). We can thus simplify handleDynamicList() by unifying the DSO and
  executable cases. It also allows us to simplify isPreemptible - now
the field is only used in finalizeSections() and later stages.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D66091

llvm-svn: 368659