gccrs: Handle tail expression normalization right before lowering to HIR.

This allows braced macros at the end of blocks to correctly expand to
zero or more statements followed by a tail expression. Parsing still
creates a tail expression for now.

gcc/rust/ChangeLog:

	* ast/rust-ast.cc (BlockExpr::strip_tail_expr):
	Don't normalize tail expression in this method.
	(BlockExpr::normalize_tail_expr): New method that only does the normalization.
	* ast/rust-expr.h: Declare new method.
	* hir/rust-ast-lower-block.h: Normalize tail expressions on blocks before lowering.

Signed-off-by: Matthew Jasper <mjjasper1@gmail.com>

gcc/rust/ast/rust-ast.cc

@@ -4223,12 +4223,10 @@ Attribute::is_parsed_to_meta_item () const
 }
 
 void
-BlockExpr::strip_tail_expr ()
+BlockExpr::normalize_tail_expr ()
 {
-  if (expr)
+  if (!expr)
     {
-      expr = nullptr;
-
       // HACK: try to turn the last statement into a tail expression
       if (statements.size () && statements.back ()->is_expr ())
 	{

gcc/rust/ast/rust-expr.h

@@ -2488,7 +2488,9 @@ public:
   }
 
   // Removes the tail expression from the block.
-  void strip_tail_expr ();
+  void strip_tail_expr () { expr = nullptr; }
+  // Normalizes a trailing statement without a semicolon to a tail expression.
+  void normalize_tail_expr ();
 
   const std::vector<Attribute> &get_outer_attrs () const { return outer_attrs; }
   std::vector<Attribute> &get_outer_attrs () override { return outer_attrs; }

gcc/rust/hir/rust-ast-lower-block.h

@@ -33,6 +33,7 @@ public:
   static HIR::BlockExpr *translate (AST::BlockExpr *expr, bool *terminated)
   {
     ASTLoweringBlock resolver;
+    expr->normalize_tail_expr ();
     expr->accept_vis (resolver);
     if (resolver.translated != nullptr)
       {