This commit addresses PR comments (#1538)

- A method was found to initial ProcAddr in DLL_Type by creating an
array of null pointers (seems like a hack)
- Comments were added in SD_FEM.f90 and SubDyn.f90 explaining that
changes were made to satisfy the Flang compiler

modules/nwtc-library/src/NWTC_Base.f90

@@ -58,16 +58,17 @@ MODULE NWTC_Base
 
    INTEGER(IntKi), PARAMETER     :: NWTC_MAX_DLL_PROC  = 5                        !< maximum number of procedures that can be dynamically loaded from a DLL (see DLL_Type nwtc_base::dll_type)
 
+   TYPE(C_FUNPTR), PARAMETER     :: NULL_PROC_ADDR(NWTC_MAX_DLL_PROC) = C_NULL_FUNPTR  !< this is a hack so the Flang compiler will initialize ProcAddr to C_NULL_FUNPTR in DLL_Type (remove if no longer needed)
 
       !> Type definition for dynamically loaded libraries:
       !! Note that changes here may need to be reflected in DLLTypePack() (nwtc_io::dlltypepack) DLLTypeUnPack() (nwtc_io::dlltypeunpack), 
       !! and the FAST Registry executable.
-   
+
    TYPE DLL_Type 
 
       INTEGER(C_INTPTR_T)       :: FileAddr  = INT(0,C_INTPTR_T)                   !< The address of file FileName.         (RETURN value from LoadLibrary ) [Windows]
       TYPE(C_PTR)               :: FileAddrX = C_NULL_PTR                          !< The address of file FileName.         (RETURN value from dlopen ) [Linux]
-      TYPE(C_FUNPTR)            :: ProcAddr(NWTC_MAX_DLL_PROC)                     !< The address of procedure ProcName.    (RETURN value from GetProcAddress or dlsym) [initialized to Null for pack/unpack]
+      TYPE(C_FUNPTR)            :: ProcAddr(NWTC_MAX_DLL_PROC) = NULL_PROC_ADDR    !< The address of procedure ProcName.    (RETURN value from GetProcAddress or dlsym) [initialized to Null for pack/unpack]
 
       CHARACTER(1024)           :: FileName                                        !< The name of the DLL file including the full path to the current working directory.
       CHARACTER(1024)           :: ProcName(NWTC_MAX_DLL_PROC)  = ""               !< The name of the procedure in the DLL that will be called.

modules/subdyn/src/SD_FEM.f90

@@ -1790,7 +1790,7 @@ SUBROUTINE DirectElimination(Init, p, ErrStat, ErrMsg)
    ! --- DOF elimination for system matrices and RHS vector
    nDOF = p%nDOF_red
    if (p%reduced) then
-      ! Temporary backup of M and K of full system
+      ! Temporary backup of M and K of full system (Flang compiler failed when move_alloc was used here, so arrays are allocated, moved, and deallocated manually)
       CALL AllocAry(KK, size(Init%K,1), size(Init%K,2), 'KK',  ErrStat2, ErrMsg2); if(Failed()) return; ! system stiffness matrix 
       CALL AllocAry(MM, size(Init%M,1), size(Init%M,2), 'MM',  ErrStat2, ErrMsg2); if(Failed()) return; ! system mass matrix 
       KK = Init%K

modules/subdyn/src/SubDyn.f90

@@ -1085,7 +1085,7 @@ SUBROUTINE SD_Input(SDInputFile, Init, p, ErrStat,ErrMsg)
 ! Reading reaction lines one by one, allowing for 1, 7 or 8 columns, with col8 being a string for the SSIfile
 do I = 1, p%nNodes_C
    READ(UnIn, FMT='(A)', IOSTAT=ErrStat2) Line; ErrMsg2='Error reading reaction line'; if (Failed()) return
-   j = index(line, achar(13))
+   j = index(line, achar(13))       ! Remove any carriage returns in this line (required by the Flang compiler)
    do while (j > 0)
       line(j:j) = " "
       j = index(line, achar(13))
@@ -1128,7 +1128,7 @@ SUBROUTINE SD_Input(SDInputFile, Init, p, ErrStat,ErrMsg)
 ! Reading interface lines one by one, allowing for 1 or 7 columns (cannot use ReadIAry)
 DO I = 1, p%nNodes_I
    READ(UnIn, FMT='(A)', IOSTAT=ErrStat2) Line  ; ErrMsg2='Error reading interface line'; if (Failed()) return
-   j = index(line, achar(13))
+   j = index(line, achar(13))    ! Remove any carriage returns in this line (required by the Flang compiler)
    do while (j > 0)
       line(j:j) = " "
       j = index(line, achar(13))