Fix a couple of class template argument deduction crashes with libc++'s tuple.

RecursiveASTVisitor was not properly recursing through a
SubstTemplateTypeParmTypes, resulting in crashes in pack expansion where we
couldn't always find an unexpanded pack within a pack expansion.

We also have an issue where substitution of deduced template arguments for an
implicit deduction guide creates the "impossible" case of naming a
non-dependent member of the current instantiation, but within a specialization
that is actually instantiated from a different (partial/explicit)
specialization of the template. We resolve this by declaring that constructors
that do so can only be used to deduce specializations of the primary template.
I'm running this past CWG to see if people agree this is the right thing to do.

llvm-svn: 304862