SI-7498 ParTrieMap.foreach no longer crashes

Previously, the `split` method of the `ParTrieMap` iterator threw
an exception when splitting a splitter that iterated over nodes
whose hash codes collide.
This was due to reusing the iterator of the list of colliding keys
rather than creating a new splitter.

This commit changes the `subdivide` method to create a new
iterator using the factory method of the current trie map
iterator rather than returning a `LinearSeqLike` iterator.
SI-7264 Initialize owner when searching for companion.

From ClassSymbol:

    protected final def companionModule0: Symbol =
      flatOwnerInfo.decl(name.toTermName).suchThat(sym => sym.isModuleNotMethod && (sym isCoDefinedWith this))

    protected final def flatOwnerInfo: Type = {
      if (needsFlatClasses)
        info
      owner.rawInfo
    }

Note the call to `rawInfo`; in the enclosed test case, that gives
us back an uninitialized type for the module class of `Foo`, and
consequently we don't find the companion for `Foo.Values`.

This commit forces the initialization of the owning symbol if it
was compiled in a prior run.

In addition, it adds a special case to `Run#compiles` for early
initialized symbols, which start out in life with the wrong owner.
As best as I can see, that complexity stems from allowing early
initialized members *without* return types to be used as value arguments
to the super call, which in turn is needed to infer parent type arguments.
The situation is described a little further in existing comments of
`typedPrimaryConstrBody`.

This bug is essentially another case of SI-6976. See the comments in pull
request of that patch (https://github.com/scala/scala/pull/1910) for
commit archaeology that shows why we're reluctant to force the owner
info more broadly than is done in this commit.
Document exceptions thrown in some conversion methods of String
Fix incorrectly documented exception of toBoolean method of String
Merge pull request #2610 from axel22/issue/7498

SI-7498 ParTrieMap.foreach no longer crashes
Merge pull request #2640 from jedesah/patch-1

Document exceptions thrown in some conversion methods of String
Merge pull request #2621 from retronym/ticket/7264

SI-7264 Initialize owner when searching for companion.