Switch to prebuilt clang-3217047

Bug: http://b/30421084
Change-Id: I8b65669ef9f60f85202295de117d3806d3a5b1f6
Test: Tested build, boot and common usage for several devices in AOSP
and internal branch.

cc/config/global.go

@@ -121,7 +121,7 @@ func init() {
 		if override := config.(android.Config).Getenv("LLVM_PREBUILTS_VERSION"); override != "" {
 			return override, nil
 		}
-		return "clang-3016494", nil
+		return "clang-3217047", nil
 	})
 	pctx.StaticVariable("ClangPath", "${ClangBase}/${HostPrebuiltTag}/${ClangVersion}")
 	pctx.StaticVariable("ClangBin", "${ClangPath}/bin")