[flang] Add runtime interface for SYSTEM_CLOCK

[rovka]

Summary

SYSTEM_CLOCK may take up to 3 optional parameters, all of which are
INTENT(OUT). The COUNT and COUNT_MAX parameters are integer scalars,
while COUNT_RATE may be a real or integer scalar.

This patch breaks up the interface into 3 different functions, one for
each parameter. All 3 return integers. It is up to lowering to convert
the results to the preferred type.

[klausler]

SYSTEM_CLOCK is a mess. Its arguments are defined as integer and real, but not *default* integer or real, so there could be 80 different ways to call it in practice.

I think the runtime API should be broken up into three distinct APIs, one for each optional argument, and defined as functions returning 64-bit integers. Lowering can convert the results.

[rovka]

Split into 3 functions.

flang/runtime/time-intrinsic.h

 
 namespace Fortran::runtime {
 extern "C" {
 
 // Lowering may need to cast this result to match the precision of the default
 // real kind.
 double RTNAME(CpuTime)();
 
+// Interface for the SYSTEM_CLOCK intrinsic. We break it up into 3 distinct
+// function calls, one for each of SYSTEM_CLOCK's optional output arguments.
+// Lowering will have to cast the results to whatever type it prefers.
+CppTypeFor<TypeCategory::Integer, 8> RTNAME(SystemClockCount)();
+CppTypeFor<TypeCategory::Integer, 8> RTNAME(SystemClockCountRate)();
+CppTypeFor<TypeCategory::Integer, 8> RTNAME(SystemClockCountMax)();
 } // extern "C"
 } // namespace Fortran::runtime
 #endif // FORTRAN_RUNTIME_TIME_INTRINSIC_H_