Allow using /main with top-level statements (#79577)

* Allow using `/main` with top-level statements

* Verify output in a pre-existing test

Author: jjonescz

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -6575,9 +6575,6 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_TopLevelStatementAfterNamespaceOrType" xml:space="preserve">
     <value>Top-level statements must precede namespace and type declarations.</value>
   </data>
-  <data name="ERR_SimpleProgramDisallowsMainType" xml:space="preserve">
-    <value>Cannot specify /main if there is a compilation unit with top-level statements.</value>
-  </data>
   <data name="ERR_SimpleProgramNotAnExecutable" xml:space="preserve">
     <value>Program using top-level statements must be an executable.</value>
   </data>

src/Compilers/CSharp/Portable/Compilation/CSharpCompilation.cs

@@ -1899,15 +1899,6 @@ internal EntryPoint GetEntryPointAndDiagnostics(CancellationToken cancellationTo
                         var entryPointMethod = FindEntryPoint(simpleProgramEntryPointSymbol, cancellationToken, out diagnostics);
                         entryPoint = new EntryPoint(entryPointMethod, diagnostics);
                     }
-
-                    if (this.Options.MainTypeName != null && simpleProgramEntryPointSymbol is object)
-                    {
-                        var diagnostics = DiagnosticBag.GetInstance();
-                        diagnostics.Add(ErrorCode.ERR_SimpleProgramDisallowsMainType, NoLocation.Singleton);
-                        entryPoint = new EntryPoint(entryPoint.MethodSymbol,
-                                                    new ReadOnlyBindingDiagnostic<AssemblySymbol>(
-                                                        entryPoint.Diagnostics.Diagnostics.Concat(diagnostics.ToReadOnlyAndFree()), entryPoint.Diagnostics.Dependencies));
-                    }
                 }
 
                 Interlocked.CompareExchange(ref _lazyEntryPoint, entryPoint, null);

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -1815,7 +1815,7 @@ internal enum ErrorCode
         ERR_SimpleProgramLocalIsReferencedOutsideOfTopLevelStatement = 8801,
         ERR_SimpleProgramMultipleUnitsWithTopLevelStatements = 8802,
         ERR_TopLevelStatementAfterNamespaceOrType = 8803,
-        ERR_SimpleProgramDisallowsMainType = 8804,
+        // ERR_SimpleProgramDisallowsMainType = 8804,
         ERR_SimpleProgramNotAnExecutable = 8805,
 
         ERR_UnsupportedCallingConvention = 8806,

src/Compilers/CSharp/Portable/Errors/ErrorFacts.cs

@@ -2074,7 +2074,6 @@ or ErrorCode.ERR_PartialMemberExtendedModDifference
                 or ErrorCode.ERR_SimpleProgramLocalIsReferencedOutsideOfTopLevelStatement
                 or ErrorCode.ERR_SimpleProgramMultipleUnitsWithTopLevelStatements
                 or ErrorCode.ERR_TopLevelStatementAfterNamespaceOrType
-                or ErrorCode.ERR_SimpleProgramDisallowsMainType
                 or ErrorCode.ERR_SimpleProgramNotAnExecutable
                 or ErrorCode.ERR_UnsupportedCallingConvention
                 or ErrorCode.ERR_InvalidFunctionPointerCallingConvention