[flang] Improve runtime interface with C99 complex

Follow up of https://reviews.llvm.org/D83397.

In folding, make pgmath usage conditional to C99 complex
support in C++. Disable warning in such case.

In lowering, use an empty class type to indicate C99 complex
type in runtime interface.

Add a unit test enforcing C99 complex can be processed
by FIR runtime interface builder.

Differential Revision: https://reviews.llvm.org/D110860