[flang] Catch error: COMMON block and implicit interface external subprogram with same name

[klausler]

Summary

Declaration checking catches the error of a COMMON block and a subprogram
definition having the same name, but misses the case of a COMMON block
and an a reference to an external subprogram with an implicit interface.
Fix. Fixes bug https://github.com/llvm/llvm-project/issues/63247.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Semantics/check-declarations.cpp

if (const auto *module{symbol.detailsIf<ModuleDetails>()}) {
     if (!module->isSubmodule() && !symbol.owner().IsIntrinsicModules()) {
       return symbol.name().ToString();
     }
   } else if (IsBlockData(symbol)) {
     return symbol.name().ToString();
   } else {
     const std::string *bindC{symbol.GetBindName()};
     if (symbol.has<CommonBlockDetails>() ||
-        IsExternalProcedureDefinition(symbol)) {
+        IsExternalProcedureDefinition(symbol) ||
+        (symbol.owner().IsGlobal() && IsExternal(symbol))) {
       return bindC ? *bindC : symbol.name().ToString();
     } else if (bindC &&
         (symbol.has<ObjectEntityDetails>() || IsModuleProcedure(symbol))) {
       return *bindC;
     }
   }
   return std::nullopt;
 }

flang/test/Semantics/declarations04.f90

contains
   subroutine bar() bind(c,name="ext3")
   end
 end
 
 block data ext3
   !PORTABILITY: Global name 'ext4' conflicts with a module
   common /ext4/ x
 end
+
+subroutine s
+  !ERROR: Two entities have the same global name 'foo'
+  common /foo/n
+  call foo
+end