[flang] Added missing _Complex for COMPLEX(10) C reduction API.

[vzakhari]

Summary

I noticed _Complex was missing for the long double case.
I might be missing something, but it looks like the compiler
does not call the C wrappers directly. Do we need to keep
the complex-reduction.[ch] at all?

flang/runtime/complex-reduction.h

 
 #if defined(_MSC_VER) && !(defined(__clang_major__) && __clang_major__ >= 12)
 typedef _Fcomplex float_Complex_t;
 typedef _Dcomplex double_Complex_t;
 typedef _Lcomplex long_double_Complex_t;
 #else
 typedef float _Complex float_Complex_t;
 typedef double _Complex double_Complex_t;
-typedef long double long_double_Complex_t;
+typedef long double _Complex long_double_Complex_t;
 #endif
 
 #define REDUCTION_ARGS \
   const struct CppDescriptor *x, const char *source, int line, int dim /*=0*/, \
       const struct CppDescriptor *mask /*=NULL*/
 #define REDUCTION_ARG_NAMES x, source, line, dim, mask
 
 float_Complex_t RTNAME(SumComplex2)(REDUCTION_ARGS);