…ynamic

Member Lookup" Types.  This is a dusted off and updated version of PR13361,
which switches from DynamicMemberLookupProtocol to @dynamicMemberLookup as
was requested by the final review decision.  This also rebases it and
updates it for other changes in the compiler.

…est I missed.

per review feedback.

…sults

even when we got normal results.

…tions

on a ConcreteDeclRefExpr because we were using the wrong locator.

thanks to Mark Lacey for pointing out this case.

add a testcase that already works for derived classes overriding the
subscript.

Thanks to Doug Gregor for both testcases!

…erLookup

to String because there was no LiteralConformsTo constraint.  Also fix a problem
where we'd dig the incorrect type out of a decl, applying an archetype to the
resolved string type.  Add testcases to cover some generics cases.

Many thanks to Doug Gregor for the testcases as well as helping me out with one
of the tricky parts of this patch!

… so a type

doesn't get recomputed as the system does backtracking.

to the same form as if the user wrote the subscript out manually.