win clang: don't disable -Wself-assign

third_party/lzma_sdk/BUILD.gn

Index: third_party/lzma_sdk/BUILD.gn
diff --git a/third_party/lzma_sdk/BUILD.gn b/third_party/lzma_sdk/BUILD.gn
index ac31fc1c2dc5e6cd4bf173987a6c60e25e70578c..2afe7b4f7e307c1c0d82c0591a748184a16f4f17 100644
--- a/third_party/lzma_sdk/BUILD.gn
+++ b/third_party/lzma_sdk/BUILD.gn
@@ -53,4 +53,14 @@ static_library("lzma_sdk") {
   configs += [ "//build/config/compiler:no_chromium_code" ]
 
   public_configs = [ ":lzma_sdk_config" ]
+
+  # Must be in a config because of how GN orders flags (otherwise -Wall will
+  # appear after this, and turn it back on).
+  config("clang_warnings") {
+    if (is_clang) {
+      # Upstream uses self-assignment to avoid warnings.
+      cflags = [ "-Wno-self-assign" ]
+    }
+  }
+  configs += [ ":clang_warnings" ]
 }