Reland D66717 [ELF] Do not ICF two sections with different output sections (by SECTIONS commands)

Recommit r370635 (reverted by r371202), with one change: move addOrphanSections() before ICF.

Before, orphan sections in two different partitions may be folded and
moved to the main partition.

Now, InputSection->OutputSection assignment for orphans happens before
ICF. ICF does not fold input sections with different output sections.

With the PR43241 reproduce,
`llvm-objcopy --extract-partition libvr.so libchrome__combined.so libvr.so` => no error

Updated description:

Fixes PR39418. Complements D47241 (the non-linker-script case).

processSectionCommands() assigns input sections to output sections.
ICF is called before it, so .text.foo and .text.bar may be folded even if
their output sections are made different by SECTIONS commands.

```
markLive<ELFT>()
doIcf<ELFT>()                      // During ICF, we don't know the output sections
writeResult()
  combineEhSections<ELFT>()
  script->processSectionCommands() // InputSection -> OutputSection assignment
```

This patch splits processSectionCommands() into processSectionCommands()
and processSymbolAssignments(), and moves
processSectionCommands()/addOrphanSections() before ICF:

```
markLive<ELFT>()
combineEhSections<ELFT>()
script->processSectionCommands()
script->addOrphanSections();
doIcf<ELFT>()                      // should remove folded input sections
writeResult()
  script->processSymbolAssignments()
```

An alternative approach is to unfold a section `sec` in
processSectionCommands() when we find `sec` and `sec->repl` belong to
different output sections. I feel this patch is superior because this
can fold more sections and the decouple of
SectionCommand/SymbolAssignment gives flexibility:

* An ExprValue can't be evaluated before its section is assigned to an
  output section -> we can delete getOutputSectionVA and simplify
  another place where we had to check if the output section is null.
  Moreover, a case in linkerscript/early-assign-symbol.s can be handled
  now.
* processSectionCommands/processSymbolAssignments can be freely moved
  around.

llvm-svn: 371216