Switch back to clang-4639204.

Bug: http://b/68397894
Test: build/run sailfish.
diff --git a/cc/config/global.go b/cc/config/global.go
index a7a2ea4..528db36 100644
--- a/cc/config/global.go
+++ b/cc/config/global.go
@@ -116,7 +116,7 @@
 
 	// prebuilts/clang default settings.
 	ClangDefaultBase         = "prebuilts/clang/host"
-	ClangDefaultVersion      = "clang-4630689"
+	ClangDefaultVersion      = "clang-4639204"
 	ClangDefaultShortVersion = "6.0.1"
 
 	// Directories with warnings from Android.bp files.