[CodeGen] Fix the type of the constant that is used to zero-initialize a

flexible array member

A zero-element array type was incorrectly being used when an incomplete
array was being initialized with a non-empty initializer.

This fixes an assertion failure in AddInitializerToStaticVarDecl. See
the discussion here: https://reviews.llvm.org/D123649#4362210

Differential Revision: https://reviews.llvm.org/D151172