Make dawn_native use its own header for Dawn datatypes

The dawn.h and dawncpp.h structure definitions references dawnFoo or
dawn::Foo respectively when it should reference dawn_native::FooBase* in
dawn_native. Autogenerate files to declare the dawn_native version of
the structs and change the ProcTable generation to use it instead.

This is important to make libdawn_native a shared library because
currently it was depending on dawncpp's definition of .Get().

src/dawn_native/opengl/PipelineLayoutGL.cpp

@@ -20,7 +20,7 @@
 
 namespace dawn_native { namespace opengl {
 
-    PipelineLayout::PipelineLayout(Device* device, const dawn::PipelineLayoutDescriptor* descriptor)
+    PipelineLayout::PipelineLayout(Device* device, const PipelineLayoutDescriptor* descriptor)
         : PipelineLayoutBase(device, descriptor) {
         GLuint uboIndex = 0;
         GLuint samplerIndex = 0;