[spirv-writer] Add start of cast support.

This CL starts support for the cast expression. It adds the i32 to f32
conversion path.

Bug: tint:5
Change-Id: Id1eafc38592060c4b5b91964302d79a847e2edaf
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/20628
Reviewed-by: David Neto <dneto@google.com>
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index a47d829..61bc317 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -431,6 +431,7 @@
     writer/spirv/builder_assign_test.cc
     writer/spirv/builder_binary_expression_test.cc
     writer/spirv/builder_call_test.cc
+    writer/spirv/builder_cast_expression_test.cc
     writer/spirv/builder_constructor_expression_test.cc
     writer/spirv/builder_entry_point_test.cc
     writer/spirv/builder_function_test.cc