tint/ast: Change Function::symbol to Function::name

Function::name is an ast::Identifier.

The goal here is to have all AST nodes use an identifier instead of
symbols directly. This will greatly simplify the renamer transform,
and gives the symbol a Source location, which is helpful for
diagnostics and tooling.

Change-Id: I723a9a104668758db2cb32051efa1f6d3c105913
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/119280
Reviewed-by: James Price <jrprice@google.com>
Kokoro: Ben Clayton <bclayton@google.com>
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>

src/tint/program_builder_test.cc

diff --git a/src/tint/program_builder_test.cc b/src/tint/program_builder_test.cc
index 24e7344..02e3ba2 100644
--- a/src/tint/program_builder_test.cc
+++ b/src/tint/program_builder_test.cc
@@ -60,7 +60,7 @@
     ASSERT_EQ(inner.AST().Functions().Length(), 1u);
     ASSERT_EQ(outer.AST().Functions().Length(), 2u);
     EXPECT_EQ(inner.AST().Functions()[0], outer.AST().Functions()[0]);
-    EXPECT_EQ(outer.AST().Functions()[1]->symbol, outer.Symbols().Get("b"));
+    EXPECT_EQ(outer.AST().Functions()[1]->name->symbol, outer.Symbols().Get("b"));
     EXPECT_EQ(inner.Symbols().Get("a"), outer.Symbols().Get("a"));
     EXPECT_TRUE(inner.Symbols().Get("a").IsValid());
     EXPECT_TRUE(outer.Symbols().Get("a").IsValid());