Closes SI-6952: add correct error positions for Dynamic feature check.
SI-6976 Fix value class separate compilation crasher.

We can't guarantee that the owner of the value class
is initialized, and if it isn't, the search for the
companion module will turn up bubkis.

This is a localized fix, but I'd be suprised if there
weren't other places that suffered from the same problem.

Wouldn't it be nicer to have something like:

    // doesn't force info
    sym.raw.info
    sym.raw.companionModule

    // forces info
    sym.info
    sym.companionModule
SI-6902 Check unreachability under @unchecked

Only exhaustiveness checking should be disabled if the
scrutinee of a match as annotated as `: @unchecked`.
This was the pre-2.10.x behaviour.

This also fixes a variation of the closed ticket,
SI-6011. The exhaustiveness check is needed to
safely fallback from emitting a table switch if
duplicate cases are detected.
SI-6969, mishandling of SoftReferences in method cache.

More interesting to test than it was to fix. The soft
reference is now dereferenced once, the locally stored
underlying value ascertained to be non-null, and the
remainder of the references to the value use the local var.

The enclosed test reliably NPEs without this patch.
Merge pull request #1910 from retronym/ticket/6976

SI-6976 Fix value class separate compilation crasher.
Merge pull request #1902 from paulp/issue/6969

SI-6969, mishandling of SoftReferences in method cache.
Merge pull request #1907 from namin/si-6952

Closes SI-6952: add correct error positions for Dynamic feature check.
Merge pull request #1928 from retronym/ticket/6902

SI-6902 Check unreachability under @unchecked
Change Doc to trait to allow it to be used as a cake slice.
Change the implementation of retrieve() method.
Rename and privatize symbols in CommentFactoryBase to avoid clashes when cooking a cake.