[libc++] Use _Complex for multiplication and division of complex floating point types (#83575)

This significantly simplifies the implementation and improves the
codegen. The only downside is that th

[libc++] Use _Complex for multiplication and division of complex floating point types (#83575)

This significantly simplifies the implementation and improves the
codegen. The only downside is that the accuracy can be marginally worse,
but that is up to the compiler to decide with this change, which means
it can be controlled by compiler flags.

Differential Revision: https://reviews.llvm.org/D155312

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