Update bindgen clang version to r468909b

r468909b is the first version to contain musl libclang.so.

Bug: 216192129
Test: m checkbuild
Change-Id: If4c986e285914ca909981ea5dc972c3047c2ac7c
diff --git a/rust/bindgen.go b/rust/bindgen.go
index 0199d3a..c5a7fb3 100644
--- a/rust/bindgen.go
+++ b/rust/bindgen.go
@@ -30,7 +30,7 @@
 	defaultBindgenFlags = []string{""}
 
 	// bindgen should specify its own Clang revision so updating Clang isn't potentially blocked on bindgen failures.
-	bindgenClangVersion = "clang-r450784d"
+	bindgenClangVersion = "clang-r468909b"
 
 	_ = pctx.VariableFunc("bindgenClangVersion", func(ctx android.PackageVarContext) string {
 		if override := ctx.Config().Getenv("LLVM_BINDGEN_PREBUILTS_VERSION"); override != "" {
@@ -239,6 +239,10 @@
 		cflags = append(cflags, "-x c")
 	}
 
+	// clang-r468909b complains about the -x c in the flags in clang-sys parse_search_paths:
+	// 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")