layers: CmdClearAttachments colorAttachment is index into subpass

We were previously treating this as being an index into the renderpass
attachment descriptions, but that's nonsense. It's also well-defined
what happens if an out-of-range index is provided, so downgrade to a
warning.

Signed-off-by: Chris Forbes <chrisforbes@google.com>
diff --git a/tests/layer_validation_tests.cpp b/tests/layer_validation_tests.cpp
index 579f8ed..ce584de 100644
--- a/tests/layer_validation_tests.cpp
+++ b/tests/layer_validation_tests.cpp
@@ -11756,9 +11756,8 @@
     TEST_DESCRIPTION("Points to a wrong colorAttachment index in a VkClearAttachment "
                      "structure passed to vkCmdClearAttachments");
     ASSERT_NO_FATAL_FAILURE(InitState());
-    m_errorMonitor->SetDesiredFailureMsg(VK_DEBUG_REPORT_ERROR_BIT_EXT,
-                                         "vkCmdClearAttachments() attachment index 1 not found in attachment "
-                                         "reference array of active subpass 0");
+    m_errorMonitor->SetDesiredFailureMsg(VK_DEBUG_REPORT_WARNING_BIT_EXT,
+                                         "vkCmdClearAttachments() color attachment index 1 out of range for active subpass 0; ignored");
 
     VKTriangleTest(bindStateVertShaderText, bindStateFragShaderText, BsoFailCmdClearAttachments);
     m_errorMonitor->VerifyFound();