On linux, clang is unable to find the Cast() function used in the
templated destroy() function unless it has already been defined
before the template function. This forces us to make sure all
Cast() functions are available, but simply adding the vulkan objects'
header files in VkMemory.h would cause a circular dependency, which,
while it would be properly guarded by the preprocessor directives,
wouldn't guarantee any include order, due to the nature of circular
dependencies. So, to fix the issue, a new header file, called
VkDestroy.h was added, which can depend on all vulkan objects' header
files without creating a circular dependency.

Also fixed some warnings.

Change-Id: I1f343a8c476d6308d4555009848a234b0695661e
Reviewed-on: https://swiftshader-review.googlesource.com/c/21668Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>