Style improvements for generated p/invokes (#69638)

- Remove '_gen` part of suffix
- Remove unnecessary braces in fixed block

src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/StubCodeContext.cs

@@ -91,7 +91,7 @@ public enum Stage
  /// </summary>
  public StubCodeContext? ParentContext { get; protected init; }
 
- public const string GeneratedNativeIdentifierSuffix = "_gen_native";
+ public const string GeneratedNativeIdentifierSuffix = "_native";
 
  /// <summary>
  /// Get managed and native instance identifiers for the <paramref name="info"/>

src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/SyntaxExtensions.cs

@@ -19,16 +19,31 @@ private static FixedStatementSyntax AddStatementWithoutEmptyStatements(this Fixe
  {
  return fixedStatement.WithStatement(childStatement);
  }
+
+ BlockSyntax block;
  if (fixedStatement.Statement.IsKind(SyntaxKind.Block))
  {
- var block = (BlockSyntax)fixedStatement.Statement;
+ block = (BlockSyntax)fixedStatement.Statement;
  if (block.Statements.Count == 0)
  {
  return fixedStatement.WithStatement(childStatement);
  }
- return fixedStatement.WithStatement(block.AddStatements(childStatement));
  }
- return fixedStatement.WithStatement(SyntaxFactory.Block(fixedStatement.Statement, childStatement));
+ else
+ {
+ block = SyntaxFactory.Block(fixedStatement.Statement);
+ }
+
+ if (childStatement.IsKind(SyntaxKind.Block))
+ {
+ block = block.WithStatements(block.Statements.AddRange(((BlockSyntax)childStatement).Statements));
+ }
+ else
+ {
+ block = block.AddStatements(childStatement);
+ }
+
+ return fixedStatement.WithStatement(block);
  }
 
  public static StatementSyntax NestFixedStatements(this ImmutableArray<FixedStatementSyntax> fixedStatements, StatementSyntax innerStatement)