[spirv-reader] Support Undef, mapping to null

Bug: tint:3
Change-Id: I1549f0445c92312b0d20292ff9d60736300d5378
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/23600
Reviewed-by: dan sinclair <dsinclair@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index bc65b11..39d66df 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -656,6 +656,7 @@
     "src/reader/spirv/function_glsl_std_450_test.cc",
     "src/reader/spirv/function_logical_test.cc",
     "src/reader/spirv/function_memory_test.cc",
+    "src/reader/spirv/function_misc_test.cc",
     "src/reader/spirv/function_var_test.cc",
     "src/reader/spirv/namer_test.cc",
     "src/reader/spirv/parser_impl_convert_member_decoration_test.cc",