Reland "Rename GrStencilAttachment class to generic GrAttachment"

This is a reland of 6113d50ec48be049c13a6e4bbea191df93e68674

Original change's description:
> Rename GrStencilAttachment class to generic GrAttachment
>
> Additional this adds a UsageFlags member to the new GrAttachment
> class.
>
> Bug: skia:10727
> Change-Id: Ifc0bfffd959f5fbc46bfcdf269e1b2a933929753
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/323107
> Commit-Queue: Greg Daniel <egdaniel@google.com>
> Reviewed-by: Brian Salomon <bsalomon@google.com>

Bug: skia:10727
Change-Id: Ie0ff0885e01c9f0666fb0cfaa765e463dcc6d0a5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/324277
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
diff --git a/src/gpu/GrRenderTargetContext.cpp b/src/gpu/GrRenderTargetContext.cpp
index c5dcb39..c8f0c91 100644
--- a/src/gpu/GrRenderTargetContext.cpp
+++ b/src/gpu/GrRenderTargetContext.cpp
@@ -25,6 +25,7 @@
 #include "src/core/SkRRectPriv.h"
 #include "src/core/SkSurfacePriv.h"
 #include "src/gpu/GrAppliedClip.h"
+#include "src/gpu/GrAttachment.h"
 #include "src/gpu/GrAuditTrail.h"
 #include "src/gpu/GrBlurUtils.h"
 #include "src/gpu/GrCaps.h"
@@ -42,7 +43,6 @@
 #include "src/gpu/GrRenderTarget.h"
 #include "src/gpu/GrRenderTargetContextPriv.h"
 #include "src/gpu/GrResourceProvider.h"
-#include "src/gpu/GrStencilAttachment.h"
 #include "src/gpu/GrStyle.h"
 #include "src/gpu/GrTracing.h"
 #include "src/gpu/SkGr.h"