[flang] Replace negative known CHARACTER length with zero in type analysis

[klausler]

Summary

When a DynamicType for CHARACTER has a known length, correct a negative
length value to its effective length of zero so that all such types
compare equal in interface compatibility checking.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/include/flang/Evaluate/type.h

     CHECK(IsValidKindOfIntrinsicType(category_, kind_));
   }
   DynamicType(int charKind, const semantics::ParamValue &len);
+  // When a known length is presented, resolve it to its effective
+  // length of zero if it is negative.
   constexpr DynamicType(int k, std::int64_t len)
-      : category_{TypeCategory::Character}, kind_{k}, knownLength_{len} {
+      : category_{TypeCategory::Character}, kind_{k}, knownLength_{
+                                                          len >= 0 ? len : 0} {
     CHECK(IsValidKindOfIntrinsicType(category_, kind_));
   }
   explicit constexpr DynamicType(

flang/lib/Evaluate/type.cpp

     : category_{TypeCategory::Character}, kind_{k} {
   CHECK(IsValidKindOfIntrinsicType(category_, kind_));
   if (auto n{ToInt64(pv.GetExplicit())}) {
-    knownLength_ = *n;
+    knownLength_ = *n > 0 ? *n : 0;
   } else {
     charLengthParamValue_ = &pv;
   }

flang/test/Semantics/separate-mp02.f90

     real :: x
   end
 end
+
+module m9
+  interface
+    module subroutine sub1(s)
+      character(len=0) s
+    end subroutine
+    module subroutine sub2(s)
+      character(len=0) s
+    end subroutine
+  end interface
+end module
+
+submodule(m9) sm1
+ contains
+  module subroutine sub1(s)
+    character(len=-1) s ! ok
+  end subroutine
+  module subroutine sub2(s)
+    !ERROR: Dummy argument 's' has type CHARACTER(KIND=1,LEN=1_8); the corresponding argument in the interface body has type CHARACTER(KIND=1,LEN=0_8)
+    character(len=1) s
+  end subroutine
+end submodule