Disable -Wdeprecated-copy with tip-of-tree Clang

while we figure out how to handle this new warning.

TBR=rnk

Bug: 1028110
Change-Id: I88be5c763bd0f2fcb40ebecc20990f96853d70c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1932804Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718651}

build/config/compiler/BUILD.gn

@@ -1549,6 +1549,13 @@ config("default_warnings") {
           # TODO(https://crbug.com/995993): Clean up and enable.
           "-Wno-implicit-fallthrough",
         ]
+
+        if (llvm_force_head_revision) {
+          cflags += [
+            # TODO(https://crbug.com/1028110): Evaluate and possible enable.
+            "-Wno-deprecated-copy",
+          ]
+        }
       }
     }
   }