[clang][RISCV] Fix incorrect ABI lowering for inherited structs under hard-float ABIs

The hard float ABIs have a rule that if a flattened struct contains
either a single fp value, or an int+fp, or fp+fp then it may be passed
in a pair of registers (if sufficient GPRs+FPRs are available).
detectFPCCEligibleStruct and the helper it calls,
detectFPCCEligibleStructHelper examine the type of the argument/return
value to determine if it complies with the requirements for this ABI
rule.

As reported in bug #57084, this logic produces incorrect results for C++
structs that inherit from other structs. This is because only the fields
of the struct were examined, but enumerating RD->fields misses any
fields in inherited C++ structs. This patch corrects that issue by
adding appropriate logic to enumerate any included base structs.

Differential Revision: https://reviews.llvm.org/D131677