bindgen: set -Wno-unknown-warning-option flag by default

Test: presubmit
Change-Id: Id8547c0b3263f4e3b060f8b0fce2aef8a494b1b2
diff --git a/rust/bindgen.go b/rust/bindgen.go
index 85cc220..454dd87 100644
--- a/rust/bindgen.go
+++ b/rust/bindgen.go
@@ -263,10 +263,9 @@
 	// clang: error: '-x c' after last input file has no effect [-Werror,-Wunused-command-line-argument]
 	cflags = append(cflags, "-Wno-unused-command-line-argument")
 
-	// LLVM_NEXT may contain flags that bindgen doesn't recognise. Turn off unknown flags warning.
-	if ctx.Config().IsEnvTrue("LLVM_NEXT") {
-		cflags = append(cflags, "-Wno-unknown-warning-option")
-	}
+	// The Clang version used by CXX can be newer than the one used by Bindgen, and uses warning related flags that
+	// it cannot recognize. Turn off unknown warning flags warning.
+	cflags = append(cflags, "-Wno-unknown-warning-option")
 
 	outputFile := android.PathForModuleOut(ctx, b.BaseSourceProvider.getStem(ctx)+".rs")