[flang] Fix common block size extension mistake in D109156

[jeanPerier]

Summary

https://reviews.llvm.org/D109156 did not properly update the case where
the equivalence symbol appearing in the common statement is the
"base symbol of an equivalence group" (this was the only case that previously
worked ok, and the patch broke it).

Fix this and add a test that actually uses this code path.

flang/lib/Semantics/compute-offsets.cpp

void ComputeOffsetsHelper::DoCommonBlock(Symbol &commonBlock) {
   std::size_t minSize{0};
   std::size_t minAlignment{0};
   for (auto &object : details.objects()) {
     Symbol &symbol{*object};
     DoSymbol(symbol);
     auto eqIter{equivalenceBlock_.end()};
     auto iter{dependents_.find(symbol)};
     if (iter == dependents_.end()) {
-      auto eqIter = equivalenceBlock_.find(symbol);
+      eqIter = equivalenceBlock_.find(symbol);
       if (eqIter != equivalenceBlock_.end()) {
         DoEquivalenceBlockBase(symbol, eqIter->second);
       }
     } else {
       SymbolAndOffset &dep{iter->second};
       Symbol &base{*dep.symbol};
       auto errorSite{
           commonBlock.name().empty() ? symbol.name() : commonBlock.name()};

flang/test/Semantics/offsets03.f90

module me
   integer :: i3, j3, l3(10)
   equivalence(i3, l3)
   common /common5/ i3, j3   ! CHECK: common5 size=40 offset=0: CommonBlockDetails alignment=4:
 
   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:
 end