Allow VK_INSTANCE_LAYERS to order implicit layers

Implicit layers previously were always loaded closest to the application regardless of
whether they appeared in any environment variables. This commit allows users to order
implicit layers using VK_INSTANCE_LAYERS, so that if a user has put an implicit layer in
that environment variable, that layer will be loaded respecting its order relative to other
layers in the env-var. If VK_INSTANCE_LAYERS contains just a single implicit layer, this
will cause that implicit layer to be loaded after all other implicit layers, and before any
app-enabled layers.

This commit also updates the documentation to reflect the un-deprecated nature of
VK_INSTANCE_LAYERS as well as other fixes related to this change.

Author: charles-lunarg

docs/LoaderInterfaceArchitecture.md

@@ -888,9 +888,7 @@ discovery.
         Known layers are those which are found by the loader taking into account
         default search paths and other environment variables
         (like <i>VK_LAYER_PATH</i>).
-        <br/>
-        This has replaced the older deprecated environment variable
-        <i>VK_INSTANCE_LAYERS</i>
+        </i>
     </small></td>
     <td><small>
         This functionality is only available with Loaders built with version
@@ -1045,8 +1043,7 @@ may be removed in a future loader release.
         <i>ppEnabledLayerNames</i>.
     </small></td>
     <td><small>
-        This has been deprecated by <i>VK_LOADER_LAYERS_ENABLE</i>.
-        It also overrides any layers disabled with
+        It overrides any layers disabled with
         <i>VK_LOADER_LAYERS_DISABLE</i>.
     </small></td>
     <td><small>

docs/LoaderLayerInterface.md

@@ -563,7 +563,7 @@ i.e. they do not require any external context to be enabled, will be enabled.
 
 ##### `VK_INSTANCE_LAYERS`
 
-The original `VK_INSTANCE_LAYERS` can be viewed as a special case of the new
+The original `VK_INSTANCE_LAYERS` can be viewed as a special case of
 `VK_LOADER_LAYERS_ENABLE`.
 Because of this, any layers enabled via `VK_INSTANCE_LAYERS` will be treated the
 same as layers enabled with `VK_LOADER_LAYERS_ENABLE` and will therefore