[clang][ItaniumMangle] Check SizeExpr for DependentSizedArrayType

(PR49478)

As ArrayType::ArrayType mentioned in clang/lib/AST/Type.cpp, a
DependentSizedArrayType might not have size expression because it it
used as the type of a dependent array of unknown bound with a dependent
braced initializer.

Thus, I add a check when mangling array of that type.

This should fix https://bugs.llvm.org/show_bug.cgi?id=49478

Reviewed By: Richard Smith - zygoloid

Differential Revision: https://reviews.llvm.org/D99407