layers:Add secondary CB warning

layers:Add secondary CB warning

Fixes #2237

If VK_COMMAND_BUFFER_USAGE_RENDER_PASS_CONTINUE_BIT is not set by a
secondary command buffer, then any renderPass specified in the
inheritance info will be ignored. Adding a warning if a non-NULL
renderPass is included in the inheritance info but the
VK_COMMAND_BUFFER_USAGE_RENDER_PASS_CONTINUE_BIT is not set. This is
likely an oversight by the user and the current validation error that
this case leads to (no active renderPass) doesn't directly point to the
root cause.