Revert "Switch to clang-4639204."

This reverts commit d4bc55624a74f3804cb37c1cc2ef9a88d366664e.

Reason for revert: PGO + LTO is broken with the new toolchain.

Change-Id: I5b84d933746a8facd536167c2b040fb08302790b
diff --git a/cc/config/global.go b/cc/config/global.go
index 528db36..a7a2ea4 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-4639204"
+	ClangDefaultVersion      = "clang-4630689"
 	ClangDefaultShortVersion = "6.0.1"
 
 	// Directories with warnings from Android.bp files.