[flang] implement assumed-rank in ENTRY (#96111)

With `createUnallocatedBox` utility change from #96106 , the TODO for assumed-rank in entry
can simply be lifted and test is added.

The key is that a unallocated assumed-rank descriptor is created with
rank zero in the entry where an assumed-rank dummy from some other entry
do not appear as a dummy (the symbol must still be mapped to some valid
value because the symbol could be used in code that would be unreachable
at runtime, but that the compiler must still generate).

Author: jeanPerier

flang/lib/Lower/ConvertVariable.cpp

@@ -2036,11 +2036,6 @@ void Fortran::lower::mapSymbolAttributes(
     if (isUnusedEntryDummy) {
       assert(!Fortran::semantics::IsAllocatableOrPointer(sym) &&
              "handled above");
-      // Need to add support for allocatable assumed-rank to use
-      // logic below, or to simplify it and add codegen for fir.zero
-      // !fir.box<> instead.
-      if (isAssumedRank)
-        TODO(loc, "assumed rank in ENTRY");
       // The box is read right away because lowering code does not expect
       // a non pointer/allocatable symbol to be mapped to a MutableBox.
       mlir::Type ty = converter.genType(var);

flang/test/Lower/HLFIR/assumed-rank-entry.f90

@@ -0,0 +1,28 @@
+! Test assumed-rank dummy argument that is not present in
+! all ENTRY statements.
+! RUN: bbc -emit-hlfir -allow-assumed-rank -o - %s | FileCheck %s
+
+subroutine test_main_entry(x)
+  real :: x(..)
+  interface
+  subroutine some_proc(x)
+    real :: x(..)
+  end subroutine
+  end interface
+  call some_proc(x)
+entry test_alternate_entry()
+  call the_end()
+end subroutine
+! CHECK-LABEL:   func.func @_QPtest_main_entry(
+! CHECK-SAME:                                  %[[VAL_0:.*]]: !fir.box<!fir.array<*:f32>> {fir.bindc_name = "x"}) {
+! CHECK:           %[[VAL_1:.*]] = fir.dummy_scope : !fir.dscope
+! CHECK:           %[[VAL_2:.*]]:2 = hlfir.declare %[[VAL_0]] dummy_scope %[[VAL_1]] {uniq_name = "_QFtest_main_entryEx"} : (!fir.box<!fir.array<*:f32>>, !fir.dscope) -> (!fir.box<!fir.array<*:f32>>, !fir.box<!fir.array<*:f32>>)
+
+! CHECK-LABEL:   func.func @_QPtest_alternate_entry() {
+! CHECK:           %[[VAL_0:.*]] = fir.alloca !fir.box<!fir.heap<!fir.array<*:f32>>>
+! CHECK:           %[[VAL_1:.*]] = fir.zero_bits !fir.heap<f32>
+! CHECK:           %[[VAL_2:.*]] = fir.embox %[[VAL_1]] : (!fir.heap<f32>) -> !fir.box<!fir.heap<f32>>
+! CHECK:           %[[VAL_3:.*]] = fir.convert %[[VAL_2]] : (!fir.box<!fir.heap<f32>>) -> !fir.box<!fir.heap<!fir.array<*:f32>>>
+! CHECK:           fir.store %[[VAL_3]] to %[[VAL_0]] : !fir.ref<!fir.box<!fir.heap<!fir.array<*:f32>>>>
+! CHECK:           %[[VAL_4:.*]] = fir.load %[[VAL_0]] : !fir.ref<!fir.box<!fir.heap<!fir.array<*:f32>>>>
+! CHECK:           %[[VAL_5:.*]]:2 = hlfir.declare %[[VAL_4]] {uniq_name = "_QFtest_main_entryEx"} : (!fir.box<!fir.heap<!fir.array<*:f32>>>) -> (!fir.box<!fir.heap<!fir.array<*:f32>>>, !fir.box<!fir.heap<!fir.array<*:f32>>>)