[flang] Mangle main program's symbol name to make it distinct from the other symbols

The following code is now accepted:
```
module m
end
program m
use m
end
```
The PROGRAM name doesn't really have an effect on the compilation result,
so it shouldn't result in symbol name conflicts.

Author: eugeneepshteyn

flang/include/flang/Semantics/symbol.h

@@ -107,7 +107,14 @@ class ModuleDetails : public WithOmpDeclarative {
 
 class MainProgramDetails : public WithOmpDeclarative {
 public:
+  MainProgramDetails() {}
+  MainProgramDetails(const SourceName &name, const SourceName &scopeName)
+  : name_{name}, scopeName_{scopeName} {}
+  const SourceName& scopeName() const { return scopeName_; }
+  const SourceName& name() const { return name_; }
 private:
+  SourceName name_; // Original main program symbol name
+  SourceName scopeName_; // Main program symbol name used in scope symbol table
 };
 
 class WithBindName {
@@ -828,7 +835,15 @@ class Symbol {
   using Flags = common::EnumSet<Flag, Flag_enumSize>;
 
   const Scope &owner() const { return *owner_; }
-  const SourceName &name() const { return name_; }
+
+  const SourceName &name() const {
+    if (const auto* details = detailsIf<MainProgramDetails>()) {
+      // For main program symbol always return the original name
+      return details->name();
+    }
+    return name_;
+  }
+
   Attrs &attrs() { return attrs_; }
   const Attrs &attrs() const { return attrs_; }
   Attrs &implicitAttrs() { return implicitAttrs_; }

flang/lib/Semantics/resolve-names.cpp

@@ -599,6 +599,18 @@ class ScopeHandler : public ImplicitRulesVisitor {
     // Note: don't use FindSymbol here. If this is a derived type scope,
     // we want to detect whether the name is already declared as a component.
     auto *symbol{FindInScope(name)};
+    if constexpr (std::is_same_v<MainProgramDetails, D>) {
+      CHECK(!symbol || !symbol->has<MainProgramDetails>());
+      // Use mangled main program name from MainProgramDetails, in order
+      // to avoid conflicts with the other kinds of symbols having the same
+      // name.
+      symbol = FindInScope(details.scopeName());
+      if (!symbol) {
+        auto &result{MakeSymbol(details.scopeName(), attrs)};
+        result.set_details(std::move(details));
+        return result;
+      }
+    }
     if (!symbol) {
       symbol = &MakeSymbol(name, attrs);
       symbol->set_details(std::move(details));
@@ -10273,10 +10285,13 @@ void ResolveNamesVisitor::AddSubpNames(ProgramTree &node) {
 bool ResolveNamesVisitor::BeginScopeForNode(const ProgramTree &node) {
   switch (node.GetKind()) {
     SWITCH_COVERS_ALL_CASES
-  case ProgramTree::Kind::Program:
+  case ProgramTree::Kind::Program: {
+    SourceName name = node.name().source;
+    SourceName scopeName = context().SaveTempName(name.ToString() + "$main"s);
     PushScope(Scope::Kind::MainProgram,
-        &MakeSymbol(node.name(), MainProgramDetails{}));
+        &MakeSymbol(node.name(), MainProgramDetails{name, scopeName}));
     return true;
+  }
   case ProgramTree::Kind::Function:
   case ProgramTree::Kind::Subroutine:
     return BeginSubprogram(node.name(), node.GetSubpFlag(),