[complex] Use the much more powerful EmitCall routine to call libcalls
for complex math.

This should fix the windows build bots that started having trouble here
and generally fix complex libcall emission on targets which use sret for
complex data types. It also makes the code a bit simpler (despite
calling into a much more complex bucket of code).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219565 91177308-0d34-0410-b5e6-96231b3b80d8