[Clang] Correctly expand pack in binary subscript expression.

When constructing an array expression where the index expression
was a pack expansion, we would construct an ArraySubscriptExpr
instead of an CreateOverloadedArraySubscriptExpr, and pack
expansion would not occur - leading a crash during code gen
or a failure during constant evaluation

Reviewed By: erichkeane, shafik

Differential Revision: https://reviews.llvm.org/D149637