Segfault caused by apparent error in codegen for exception unwinding on s390x

On s390x, when this call to parseElement results in an exception, exception unwinding triggers a call to the FieldPath destructor, but that destructor gets called on address 0x0. Inspection of the disassembly reveals that the exception unwinding code is incorrectly attempting to read the address of the FieldPath object from a volatile (caller-save) register, which has been clobbered with a 0 value.

As a workaround, we can assign the FieldPath to a named variable, instead of constructing it within the argument list to ProjectTypeParser::parseElement().