swift234

Mixing lvalues and rvalues in tuple exprs is very unhappy.

In SR-628 in particular, the problem was an assert that an AccessKind
was being set on a non-lvalue, but there were lots of asserts here in
various scenarios, the most common other ones being an AccessKind not
being set assertion by the ASTVerifier or lvalue-ness not matching
between tuple expr and tuple element expr.

This checks for lvalues in the tuple when a tuple indexing expr is built,
and if there are any, inserts load exprs into the lvalue elements to make all rvalues.