Enable ClangTidy check llvm-namespace-comment.

This fixes a large number of SkSL namespaces which were labeled as if
they were anonymous, and also a handful of other mislabeled namespaces.
Missing namespace-end comments have been added throughout.
A number of diffs are just indentation-related (adjusting 1- or 3-
space indents to 2-space).

Change-Id: I6c62052a0d3aea4ae12ca07e0c2a8587b2fce4ec
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/308503
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
diff --git a/samplecode/SampleThinAA.cpp b/samplecode/SampleThinAA.cpp
index d7499ba..5608acb 100644
--- a/samplecode/SampleThinAA.cpp
+++ b/samplecode/SampleThinAA.cpp
@@ -542,4 +542,4 @@
 
 DEF_SAMPLE( return new ThinAASample; )
 
-}
+}  // namespace skiagm