Revert "[flang] IEEE_ARITHMETIC must imply USE IEEE_EXCEPTIONS"

This reverts commit b6a7600. It caused
the following build failure:
```
ninja: error: dependency cycle: include/flang/__fortran_ieee_exceptions.mod -> include/flang/__fortran_ieee_exceptions.mod
```

See e.g.:
* https://lab.llvm.org/buildbot/#/builders/172/builds/9595

To reproduce:
```
cmake -G Ninja \
  -DLLVM_TARGETS_TO_BUILD=host \
  -DCMAKE_BUILD_TYPE=Release \
  -DLLVM_ENABLE_PROJECTS="clang;flang" \
  ../../llvm
ninja check-flang
```

Author: banach-space

flang/lib/Semantics/check-do-forall.cpp

@@ -279,8 +279,7 @@ class DoConcurrentBodyEnforce {
             " CONCURRENT"_err_en_US,
             doConcurrentSourcePosition_);
       }
-      if (name->symbol &&
-          fromScope(*name->symbol, "__fortran_ieee_exceptions"s)) {
+      if (name->symbol && fromScope(*name->symbol, "ieee_exceptions"s)) {
         if (name->source == "ieee_set_halting_mode") {
           SayWithDo(context_, currentStatementSourcePosition_,
               "IEEE_SET_HALTING_MODE is not allowed in DO "

flang/module/__fortran_ieee_exceptions.f90

@@ -29,11 +29,6 @@ module ieee_arithmetic
     ieee_support_subnormal => __builtin_ieee_support_subnormal, &
     ieee_support_underflow_control => __builtin_ieee_support_underflow_control
 
-  ! 17.1: "The module IEEE_ARITHMETIC behaves as if it contained a USE statement
-  ! for IEEE_EXCEPTIONS; everything that is public in IEEE_EXCEPTIONS is public
-  ! in IEEE_ARITHMETIC."
-  use __Fortran_ieee_exceptions
-
   implicit none
 
   type :: ieee_class_type

flang/module/ieee_arithmetic.f90

@@ -6,6 +6,120 @@
 !
 !===------------------------------------------------------------------------===!
 
+! See Fortran 2018, clause 17
 module ieee_exceptions
-  use __Fortran_ieee_exceptions
+
+  type :: ieee_flag_type ! Fortran 2018, 17.2 & 17.3
+    private
+    integer(kind=1) :: flag = 0
+  end type ieee_flag_type
+
+  type(ieee_flag_type), parameter :: &
+    ieee_invalid = ieee_flag_type(1), &
+    ieee_overflow = ieee_flag_type(2), &
+    ieee_divide_by_zero = ieee_flag_type(4), &
+    ieee_underflow = ieee_flag_type(8), &
+    ieee_inexact = ieee_flag_type(16), &
+    ieee_denorm = ieee_flag_type(32) ! PGI extension
+
+  type(ieee_flag_type), parameter :: &
+    ieee_usual(*) = [ &
+      ieee_overflow, ieee_divide_by_zero, ieee_invalid ], &
+    ieee_all(*) = [ &
+      ieee_usual, ieee_underflow, ieee_inexact, ieee_denorm ]
+
+  type :: ieee_modes_type ! Fortran 2018, 17.7
+    private
+  end type ieee_modes_type
+
+  type :: ieee_status_type ! Fortran 2018, 17.7
+    private
+  end type ieee_status_type
+
+  private :: ieee_support_flag_2, ieee_support_flag_3, &
+      ieee_support_flag_4, ieee_support_flag_8, ieee_support_flag_10, &
+      ieee_support_flag_16
+  interface ieee_support_flag
+    module procedure :: ieee_support_flag, &
+      ieee_support_flag_2, ieee_support_flag_3, &
+      ieee_support_flag_4, ieee_support_flag_8, ieee_support_flag_10, &
+      ieee_support_flag_16
+  end interface
+
+ contains
+  elemental subroutine ieee_get_flag(flag, flag_value)
+    type(ieee_flag_type), intent(in) :: flag
+    logical, intent(out) :: flag_value
+  end subroutine ieee_get_flag
+
+  elemental subroutine ieee_get_halting_mode(flag, halting)
+    type(ieee_flag_type), intent(in) :: flag
+    logical, intent(out) :: halting
+  end subroutine ieee_get_halting_mode
+
+  subroutine ieee_get_modes(modes)
+    type(ieee_modes_type), intent(out) :: modes
+  end subroutine ieee_get_modes
+
+  subroutine ieee_get_status(status)
+    type(ieee_status_type), intent(out) :: status
+  end subroutine ieee_get_status
+
+  pure subroutine ieee_set_flag(flag, flag_value)
+    type(ieee_flag_type), intent(in) :: flag
+    logical, intent(in) :: flag_value
+  end subroutine ieee_set_flag
+
+  pure subroutine ieee_set_halting_mode(flag, halting)
+    type(ieee_flag_type), intent(in) :: flag
+    logical, intent(in) :: halting
+  end subroutine ieee_set_halting_mode
+
+  subroutine ieee_set_modes(modes)
+    type(ieee_modes_type), intent(in) :: modes
+  end subroutine ieee_set_modes
+
+  subroutine ieee_set_status(status)
+    type(ieee_status_type), intent(in) :: status
+  end subroutine ieee_set_status
+
+  pure logical function ieee_support_flag(flag)
+    type(ieee_flag_type), intent(in) :: flag
+    ieee_support_flag = .true.
+  end function
+  pure logical function ieee_support_flag_2(flag, x)
+    type(ieee_flag_type), intent(in) :: flag
+    real(kind=2), intent(in) :: x(..)
+    ieee_support_flag_2 = .true.
+  end function
+  pure logical function ieee_support_flag_3(flag, x)
+    type(ieee_flag_type), intent(in) :: flag
+    real(kind=3), intent(in) :: x(..)
+    ieee_support_flag_3 = .true.
+  end function
+  pure logical function ieee_support_flag_4(flag, x)
+    type(ieee_flag_type), intent(in) :: flag
+    real(kind=4), intent(in) :: x(..)
+    ieee_support_flag_4 = .true.
+  end function
+  pure logical function ieee_support_flag_8(flag, x)
+    type(ieee_flag_type), intent(in) :: flag
+    real(kind=8), intent(in) :: x(..)
+    ieee_support_flag_8 = .true.
+  end function
+  pure logical function ieee_support_flag_10(flag, x)
+    type(ieee_flag_type), intent(in) :: flag
+    real(kind=10), intent(in) :: x(..)
+    ieee_support_flag_10 = .true.
+  end function
+  pure logical function ieee_support_flag_16(flag, x)
+    type(ieee_flag_type), intent(in) :: flag
+    real(kind=16), intent(in) :: x(..)
+    ieee_support_flag_16 = .true.
+  end function
+
+  pure logical function ieee_support_halting(flag)
+    type(ieee_flag_type), intent(in) :: flag
+  end function ieee_support_halting
+
 end module ieee_exceptions

flang/module/ieee_exceptions.f90

@@ -6,7 +6,6 @@ set(LLVM_LINK_COMPONENTS
 
 set(MODULES
   "__fortran_builtins"
-  "__fortran_ieee_exceptions"
   "__fortran_type_info"
   "ieee_arithmetic"
   "ieee_exceptions"
@@ -28,10 +27,6 @@ foreach(filename ${MODULES})
     if(NOT ${filename} MATCHES "__fortran_type_info")
       set(depends ${FLANG_INTRINSIC_MODULES_DIR}/__fortran_type_info.mod)
     endif()
-    if(${filename} MATCHES "ieee_arithmetic" OR
-       ${filename} MATCHES "ieee_exceptions")
-      set(depends ${FLANG_INTRINSIC_MODULES_DIR}/__fortran_ieee_exceptions.mod)
-    endif()
   endif()
   add_custom_command(OUTPUT ${base}.mod
     COMMAND ${CMAKE_COMMAND} -E make_directory ${FLANG_INTRINSIC_MODULES_DIR}