Add BlockStatement insert and non-const global_variables

Insert will be used for adding code at the beginning of a function, and
non-const global_variables to edit decorations.

Bug: dawn:480
Change-Id: I9fbb0210a95b8488c9ce6d9a536a68f169b0cc33
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/25850
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

src/ast/block_statement.h

diff --git a/src/ast/block_statement.h b/src/ast/block_statement.h
index 4ee0806..3c2a268 100644
--- a/src/ast/block_statement.h
+++ b/src/ast/block_statement.h
@@ -42,6 +42,13 @@
     statements_.push_back(std::move(stmt));
   }
 
+  /// Insert a statement to the block
+  /// @param index the index to insert at
+  /// @param stmt the statement to insert
+  void insert(size_t index, std::unique_ptr<ast::Statement> stmt) {
+    statements_.insert(statements_.begin() + index, std::move(stmt));
+  }
+
   /// @returns true if the block is empty
   bool empty() const { return statements_.empty(); }
   /// @returns the number of statements directly in the block

src/ast/block_statement_test.cc

diff --git a/src/ast/block_statement_test.cc b/src/ast/block_statement_test.cc
index f0a6cbd..2d040d3 100644
--- a/src/ast/block_statement_test.cc
+++ b/src/ast/block_statement_test.cc
@@ -38,6 +38,27 @@
   EXPECT_EQ(b[0], ptr);
 }
 
+TEST_F(BlockStatementTest, Creation_WithInsert) {
+  auto s1 = std::make_unique<DiscardStatement>();
+  auto s2 = std::make_unique<DiscardStatement>();
+  auto s3 = std::make_unique<DiscardStatement>();
+  auto* p1 = s1.get();
+  auto* p2 = s2.get();
+  auto* p3 = s3.get();
+
+  BlockStatement b;
+  b.insert(0, std::move(s1));
+  b.insert(0, std::move(s2));
+  b.insert(1, std::move(s3));
+
+  // |b| should contain s2, s3, s1
+
+  ASSERT_EQ(b.size(), 3u);
+  EXPECT_EQ(b[0], p2);
+  EXPECT_EQ(b[1], p3);
+  EXPECT_EQ(b[2], p1);
+}
+
 TEST_F(BlockStatementTest, Creation_WithSource) {
   BlockStatement b(Source{20, 2});
   auto src = b.source();

src/ast/module.h

diff --git a/src/ast/module.h b/src/ast/module.h
index d17a0cb..7734026 100644
--- a/src/ast/module.h
+++ b/src/ast/module.h
@@ -57,6 +57,9 @@
   /// @returns the global variables for the module
   const VariableList& global_variables() const { return global_variables_; }
 
+  /// @returns the global variables for the module
+  VariableList& global_variables() { return global_variables_; }
+
   /// Adds an entry point to the module
   /// @param ep the entry point to add
   void AddEntryPoint(std::unique_ptr<EntryPoint> ep) {