[flang] Fix references to equivalenced variables in inner procedures

The code that collects that host associated variables variables in inner
procedures was seeing entities of type Fortran::lower::pft::Variable which
were created for handling Fortran variables that appear in EQUIVANCE
statements.  Such entities throw an assertion when `getSymbol() is called on
them because they're not of the `Nominal` variant.

I wrote several tests (with help from Jean and Eric) for various situations
where EQUIVALENCE and host association were combined.  In every case where
there was a host-associated Fortran variable appeared in an EQUIVALENCE
statement, there were two entities created -- an entity of the `AggregateStore`
variant and an entity of the `Nominal` variant.

So it seems safe and correct to just ignore `Variable`s of the `AggregateStore`
variant when gathering host associated entities.  So that's what I did.

I also added a FIXME to deal with the fact that internal procedures that
access variables in EQUIVALENCE sets in outer scopes unnecessarily
create their own AggregateStore.

Note that my first attempt to fix this was in PR#1165.

Author: psteinfeld

flang/lib/Lower/Bridge.cpp

@@ -214,6 +214,8 @@ class FirConverter : public Fortran::lower::AbstractConverter {
       Fortran::lower::pft::FunctionLikeUnit &funit,
       llvm::SetVector<const Fortran::semantics::Symbol *> &escapees) {
     for (const auto &var : funit.getOrderedSymbolTable()) {
+      if (var.isAggregateStore())
+        continue;
       const auto &sym = var.getSymbol();
       if (const auto *escapingSym = getIfHostProcedureSymbol(sym)) {
         LLVM_DEBUG(llvm::dbgs() << "host associated symbol " << sym << '\n');

flang/lib/Lower/PFTBuilder.cpp

@@ -1231,6 +1231,11 @@ struct SymbolDependenceDepth {
       : vars{vars}, reentrant{reentrant} {}
 
   void analyzeAliasesInCurrentScope(const semantics::Scope &scope) {
+    // FIXME: When this function is called on the scope of an internal
+    // procedure whose parent contains an EQUIVALENCE set and the internal
+    // procedure uses variables from that EQUIVALENCE set, we end up creating
+    // an AggregateStore for those variables unnecessarily.
+    //
     /// If this is a function nested in a module no host associated
     /// symbol are added to the function scope for module symbols used in this
     /// scope. As a result, alias analysis in parent module scopes must be