[flang][runtime] Replace recursion with iterative work queue

Recursion, both direct and indirect, prevents accurate stack size
calculation at link time for GPU device code.  Restructure these
recursive (often mutually so) routines in the Fortran runtime
with new implementations based on an iterative work queue with
suspendable/resumable work tickets: Assign, Initialize, initializeClone,
Finalize, Destroy, and DescriptorIO.

Note that derived type FINAL subroutine calls, defined assignments,
and defined I/O procedures all perform callbacks into user code,
which may well reenter the runtime library.  This kind of recursion
is not handled by this change, although it may be possible to do so
in the future using thread-local work queues.

The effects of this restructuring on CPU performance are yet to be
measured.

There is a fast(?) mode in the work queue implementation that causes
new work items to be executed to completion immediately upon
creation, saving the overhead of actually representing and managing
the work queue.  This mode can't be used on GPU devices, but it
is enabled by default for CPU hosts.  It can be disabled easily
for debugging and performance testing.

Author: klausler

flang-rt/include/flang-rt/runtime/environment.h

@@ -63,6 +63,7 @@ struct ExecutionEnvironment {
   bool noStopMessage{false}; // NO_STOP_MESSAGE=1 inhibits "Fortran STOP"
   bool defaultUTF8{false}; // DEFAULT_UTF8
   bool checkPointerDeallocation{true}; // FORT_CHECK_POINTER_DEALLOCATION
+  int internalDebugging{0}; // FLANG_RT_DEBUG
 
   // CUDA related variables
   std::size_t cudaStackLimit{0}; // ACC_OFFLOAD_STACK_SIZE

flang-rt/include/flang-rt/runtime/stat.h

@@ -24,7 +24,7 @@ class Terminator;
 enum Stat {
   StatOk = 0, // required to be zero by Fortran
 
-  // Interoperable STAT= codes
+  // Interoperable STAT= codes (>= 11)
   StatBaseNull = CFI_ERROR_BASE_ADDR_NULL,
   StatBaseNotNull = CFI_ERROR_BASE_ADDR_NOT_NULL,
   StatInvalidElemLen = CFI_INVALID_ELEM_LEN,
@@ -36,7 +36,7 @@ enum Stat {
   StatMemAllocation = CFI_ERROR_MEM_ALLOCATION,
   StatOutOfBounds = CFI_ERROR_OUT_OF_BOUNDS,
 
-  // Standard STAT= values
+  // Standard STAT= values (>= 101)
   StatFailedImage = FORTRAN_RUNTIME_STAT_FAILED_IMAGE,
   StatLocked = FORTRAN_RUNTIME_STAT_LOCKED,
   StatLockedOtherImage = FORTRAN_RUNTIME_STAT_LOCKED_OTHER_IMAGE,
@@ -49,10 +49,14 @@ enum Stat {
   // Additional "processor-defined" STAT= values
   StatInvalidArgumentNumber = FORTRAN_RUNTIME_STAT_INVALID_ARG_NUMBER,
   StatMissingArgument = FORTRAN_RUNTIME_STAT_MISSING_ARG,
-  StatValueTooShort = FORTRAN_RUNTIME_STAT_VALUE_TOO_SHORT,
+  StatValueTooShort = FORTRAN_RUNTIME_STAT_VALUE_TOO_SHORT, // -1
   StatMoveAllocSameAllocatable =
       FORTRAN_RUNTIME_STAT_MOVE_ALLOC_SAME_ALLOCATABLE,
   StatBadPointerDeallocation = FORTRAN_RUNTIME_STAT_BAD_POINTER_DEALLOCATION,
+
+  // Dummy status for work queue continuation, declared here to perhaps
+  // avoid collisions
+  StatContinue = 201
 };
 
 RT_API_ATTRS const char *StatErrorString(int);