Split the libdawn target in components with a single purpose.

The functionality of the dawn_headers and libdawn targets are split into
the following targets:

 - dawn_headers: the new version only exposes the "dawn.h" C API and no
   longer includes the C++ API.
 - dawncpp: the header and implementation of the C++ API that wraps the
   C API. This is unbundled from the rest so the C++ API can be used
   with libdawn_proc or other libraries implementing the C API.
 - libdawn_proc: A DawnProcTable-backend implementation of the C API.

This is needed because in follow-up commit there will be three libraries
implementing the C API: libdawn_proc that trampolines where we want, and
libdawn_native/wire that don't have trampolines for better perf.

BUG=dawn:22

Change-Id: I5d941f0d98e5a4b633e14d67eb5269f7924f0647
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/12160
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>

src/utils/BackendBinding.h

@@ -15,7 +15,7 @@
 #ifndef UTILS_BACKENDBINDING_H_
 #define UTILS_BACKENDBINDING_H_
 
-#include "dawn/dawncpp.h"
+#include "dawn/dawn.h"
 #include "dawn_native/DawnNative.h"
 
 struct GLFWwindow;