[flang] Fix spurious error with COMMON and EQUIVALENCE

[klausler]

Summary

f18 emits an error message when two objects related by EQUIVALENCE
to a third are specified as members of a COMMON block. This is not
always a sign of an error, however; it is possible for multiple objects
in a COMMON block to all be equivalenced to distinct offsets in another
object in a way that is consistent. So refine the check.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Semantics/compute-offsets.cpp

if (iter == dependents_.end()) {
       if (eqIter != equivalenceBlock_.end()) {
         DoEquivalenceBlockBase(symbol, eqIter->second);
       }
     } else {
       SymbolAndOffset &dep{iter->second};
       Symbol &base{*dep.symbol};
       if (const auto *baseBlock{FindCommonBlockContaining(base)}) {
         if (baseBlock == &commonBlock) {
+          if (base.offset() != symbol.offset() - dep.offset ||
+              std::find(details.objects().begin(), details.objects().end(),
+                  base) != details.objects().end()) {
             context_.Say(errorSite,
                 "'%s' is storage associated with '%s' by EQUIVALENCE elsewhere in COMMON block /%s/"_err_en_US,
                 symbol.name(), base.name(), commonBlock.name());
+          }
         } else { // 8.10.3(1)
           context_.Say(errorSite,
               "'%s' in COMMON block /%s/ must not be storage associated with '%s' in COMMON block /%s/ by EQUIVALENCE"_err_en_US,
               symbol.name(), commonBlock.name(), base.name(),
               baseBlock->name());
         }
       } else if (dep.offset > symbol.offset()) { // 8.10.3(3)
         context_.Say(errorSite,

flang/test/Semantics/offsets03.f90

module me
   integer :: i4, j4, l4(10), k4(10)
   equivalence(i4, l4)
   equivalence(l4(10), k4)
   common /common6/ i4, j4   ! CHECK: common6 size=76 offset=0: CommonBlockDetails alignment=4:
 
   integer :: i5, j5, l5(10)
   equivalence(l5(1), i5)
   common /common7/ j5, i5   ! CHECK: common7 size=44 offset=0: CommonBlockDetails alignment=4:
+
+  real :: a1, a2, a3(2)
+  equivalence(a1,a3(1)),(a2,a3(2))
+  common /common8/ a1, a2   ! CHECK: common8 size=8 offset=0: CommonBlockDetails alignment=4:
 end