flip on a bunch of GCC warnings

I've landed fixes for most of these,
leaving -Wclass-memaccess as a TODO.

Bug: skia:9674
Change-Id: Ifb951bc66e022b48ff4b66e4555d3fe3c7ef5aaf
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/257501
Reviewed-by: Ben Wagner aka dogben <benjaminwagner@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>

gn/BUILD.gn

@@ -351,16 +351,12 @@ config("warnings") {
       "-Wnon-virtual-dtor",
       "-Wno-noexcept-type",
 
-      # TODO(dogben): The following are new between GCC 6 (stretch) and GCC 8 (buster); they should
-      # all be investigated.
-      "-Wno-array-bounds",
+      # GCC 8+ bundles a number of fundamentally different warnings under this same flag,
+      # ranging from false positive (only copying a prefix of SkRRect from serialized form)
+      # to possibly useful (memcpy() with non-trivial types).  Annoyingly you can't really
+      # break them up any finer.
+      # TODO(mtklein): suppress / fix each site as appropriate?
       "-Wno-class-memaccess",
-      "-Wno-stringop-overflow",
-      "-Wno-restrict",
-      "-Wno-stringop-truncation",
-      "-Wno-sizeof-pointer-memaccess",
-      "-Wno-parentheses",
-      "-Wno-format-truncation",
     ]
   }