Allow the clang version used for bindgen to be configured.

This is useful when experimenting with new compiler flags that the
existing compiler does not support.

Change-Id: I45b8a2980b247d60a7f4e2ba55169267aeceef2b
diff --git a/rust/bindgen.go b/rust/bindgen.go
index 35a807b..56d660e 100644
--- a/rust/bindgen.go
+++ b/rust/bindgen.go
@@ -31,12 +31,19 @@
 	// bindgen should specify its own Clang revision so updating Clang isn't potentially blocked on bindgen failures.
 	bindgenClangVersion = "clang-r399163b"
 
+	_ = pctx.VariableFunc("bindgenClangVersion", func(ctx android.PackageVarContext) string {
+		if override := ctx.Config().Getenv("LLVM_BINDGEN_PREBUILTS_VERSION"); override != "" {
+			return override
+		}
+		return bindgenClangVersion
+	})
+
 	//TODO(b/160803703) Use a prebuilt bindgen instead of the built bindgen.
 	_ = pctx.HostBinToolVariable("bindgenCmd", "bindgen")
 	_ = pctx.SourcePathVariable("bindgenClang",
-		"${cc_config.ClangBase}/${config.HostPrebuiltTag}/"+bindgenClangVersion+"/bin/clang")
+		"${cc_config.ClangBase}/${config.HostPrebuiltTag}/${bindgenClangVersion}/bin/clang")
 	_ = pctx.SourcePathVariable("bindgenLibClang",
-		"${cc_config.ClangBase}/${config.HostPrebuiltTag}/"+bindgenClangVersion+"/lib64/")
+		"${cc_config.ClangBase}/${config.HostPrebuiltTag}/${bindgenClangVersion}/lib64/")
 
 	//TODO(ivanlozano) Switch this to RuleBuilder
 	bindgen = pctx.AndroidStaticRule("bindgen",