Automate pod arg implementation decision (#575)

Contributes to #529

* Decision made on a per-kernel basis
* New pass assigns metadata to kernels
  * passes use that metadata through new utliities to get the right
  ArgKinds
  * currently only decides based on command line options (so NFC)
* Refactored layout validation into new files
* Refactored some global push constant methods
* new enum for how pod args are implemented
  * kGlobalPushConstant will be implemented in the future
* added new metadata to test that required it
* changed arg kind utility to take the arg instead of just the type
* Priorities per-kernel push constants then ubo then ssbo
  * checks for compatibility
* Added new variant of isValidExplicitLayout that checks the whole
struct
diff --git a/lib/Passes.cpp b/lib/Passes.cpp
index dae883d..d7bc38e 100644
--- a/lib/Passes.cpp
+++ b/lib/Passes.cpp
@@ -18,6 +18,7 @@
 
 void initializeClspvPasses(PassRegistry &r) {
   initializeAddFunctionAttributesPassPass(r);
+  initializeAutoPodArgsPassPass(r);
   initializeAllocateDescriptorsPassPass(r);
   initializeClusterModuleScopeConstantVarsPass(r);
   initializeClusterPodKernelArgumentsPassPass(r);