[msl-writer] Add variable declarations.

This CL adds variable declarations to the MSL backend.

Bug: tint:7
Change-Id: Icf63ad44a217213e5036eb76429e25a3031822eb
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/24540
Reviewed-by: David Neto <dneto@google.com>
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 0776647..66ae755 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -515,6 +515,7 @@
     writer/msl/generator_impl_test.cc
     writer/msl/generator_impl_type_test.cc
     writer/msl/generator_impl_unary_op_test.cc
+    writer/msl/generator_impl_variable_decl_statement_test.cc
     writer/msl/namer_test.cc
   )
 endif()