Fix MaterializeTemporaryExpr's type when its an incomplete array.

Like the VarDecl that gets its type updated based on an init-list, this
patch corrects the MaterializeTemporaryExpr's type to make sure it isn't
creating an incomplete type, which leads to a handful of CodeGen crashes
(see PR 47636).

Based on @rsmith 's comments on D88236

Differential Revision: https://reviews.llvm.org/D88298