[flang] Catch repeated BIND(C) attribute specifications for a symbol

[klausler]

Summary

A BIND(C) attribute statement or type-declaration-stmt attribute, just
like most attributes, can only appear once. Name resolution was excluding
the BIND(C) attribute from its check for duplicated attributes, but I
don't see a reason that remains to do so.

flang/lib/Semantics/resolve-names.cpp

if (symbol.attrs().test(Attr::INTRINSIC)) { // C840
           "Symbol '%s' cannot have both EXTERNAL and INTRINSIC attributes"_err_en_US,
           symbol.name());
     }
   }
 }
 
 bool ScopeHandler::CheckDuplicatedAttr(
     SourceName name, const Symbol &symbol, Attr attr) {
-  if (attr == Attr::SAVE || attr == Attr::BIND_C) {
-    // these are checked elsewhere
+  if (attr == Attr::SAVE) {
+    // checked elsewhere
   } else if (symbol.attrs().test(attr)) { // C815
     if (symbol.implicitAttrs().test(attr)) {
       // Implied attribute is now confirmed explicitly
     } else {
       Say(name, "%s attribute was already specified on '%s'"_err_en_US,
           EnumToString(attr), name);
       return false;
     }

flang/test/Semantics/bind-c02.f90

 ! RUN: %python %S/test_errors.py %s %flang_fc1
 ! Check for 8.6.4(1)
 ! The BIND statement specifies the BIND attribute for a list of variables and
 ! common blocks.
 
 module m
 
   interface
-    subroutine proc() bind(c)
+    subroutine proc()
     end
   end interface
-  procedure(proc), bind(c) :: pc1
+  procedure(proc) :: pc1
   !ERROR: Only variable and named common block can be in BIND statement
   bind(c) :: proc
   !ERROR: Only variable and named common block can be in BIND statement
   bind(c) :: pc1
 
   !ERROR: Only variable and named common block can be in BIND statement
   bind(c) :: sub
 

flang/test/Semantics/declarations03.f90

module m
   !ERROR: Two entities have the same global name 'ff'
   common /blk6/ j, /blk7/ k
   bind(c, name="ff") :: /blk6/
   bind(c, name="ff") :: /blk7/
 
   !ERROR: The entity 's1' has multiple BIND names
   integer :: s1
   bind(c, name="gg") :: s1
+  !ERROR: BIND_C attribute was already specified on 's1'
   bind(c, name="hh") :: s1
 
   !ERROR: Two entities have the same global name 'ii'
   integer :: s2, s3
   bind(c, name="ii") :: s2
   bind(c, name="ii") :: s3
 
   !ERROR: The entity 's4' has multiple BIND names
   integer, bind(c, name="ss1") :: s4
+  !ERROR: BIND_C attribute was already specified on 's4'
   bind(c, name="jj") :: s4
 
   !ERROR: The entity 's5' has multiple BIND names
   bind(c, name="kk") :: s5
+  !ERROR: BIND_C attribute was already specified on 's5'
   integer, bind(c, name="ss2") :: s5
 
 end
 
 subroutine common1()
   real :: x
   common /com/ x
   bind(c, name='xcom') /com/ ! no error