[clang][codegen] Fix emission of consteval constructor of derived type

For simple derived type ConstantEmitter returns a struct of the same
size but different type which is then stored field-by-field into memory
via pointer to derived type. In case base type has more fields than derived,
the incorrect GEP is emitted. So, just cast pointer to derived type to
appropriate type with enough fields.

Fixes https://github.com/llvm/llvm-project/issues/60166

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D142534