[flang] Fix one regression failure related to BIND(C) statement

[peixin]

Summary

For BIND(C) statement, two common block with the same name can have the
same bind name. Fix the regression failure by add this check. Also add
the regression tests.

Co-authored-by: Jean Perier <[e-mail]>

[clementval]

LGTM. Thanks for the fix.

flang/lib/Semantics/check-declarations.cpp

void CheckHelper::CheckBindC(const Symbol &symbol) {
   if (symbol.has<ObjectEntityDetails>() && !symbol.owner().IsModule()) {
     messages_.Say(symbol.name(),
         "A variable with BIND(C) attribute may only appear in the specification part of a module"_err_en_US);
   }
   if (const std::string * name{DefinesBindCName(symbol)}) {
     auto pair{bindC_.emplace(*name, symbol)};
     if (!pair.second) {
       const Symbol &other{*pair.first->second};
-      if (DefinesBindCName(other) && !context_.HasError(other)) {
+      // Two common blocks with the same name can have the same BIND(C) name.
+      if ((!symbol.has<CommonBlockDetails>() ||
+              symbol.name() != other.name()) &&
+          DefinesBindCName(other) && !context_.HasError(other)) {
         if (auto *msg{messages_.Say(symbol.name(),
                 "Two symbols have the same BIND(C) name '%s'"_err_en_US,
                 *name)}) {
           msg->Attach(other.name(), "Conflicting symbol"_en_US);
         }
         context_.SetError(symbol);
         context_.SetError(other);
       }

flang/test/Semantics/declarations03.f90

module m
   integer, bind(c, name="ss1") :: s4
   bind(c, name="jj") :: s4
 
   !ERROR: The entity 's5' has multiple BIND names
   bind(c, name="kk") :: s5
   integer, bind(c, name="ss2") :: s5
 
 end
+
+subroutine common1()
+  real :: x
+  common /com/ x
+  bind(c, name='xcom') /com/ ! no error
+end subroutine
+
+subroutine common2()
+  real :: x
+  common /com/ x
+  bind(c, name='xcom') /com/ ! no error
+end subroutine
+
+module a
+  integer, bind(c, name="int") :: i
+end module
+
+module b
+  !ERROR: Two symbols have the same BIND(C) name 'int'
+  integer, bind(c, name="int") :: i
+end module