Bump Clang to clang-r407598b

clang-r407598b is the one currently used by AOSP:
https://android.googlesource.com/platform/prebuilts/clang/host/linux-x86/+/master/README.md

Tested locally and all tests pass.

Change-Id: I6a5e174481cf4e7f48b39d5200ccb696bbd3c507
diff --git a/Makefile b/Makefile
index 0ff98b5..98f4e88 100644
--- a/Makefile
+++ b/Makefile
@@ -69,7 +69,7 @@
 
 # Find the toolchain path for the current OS.
 UNNAME_S := $(shell uname -s | tr '[:upper:]' '[:lower:]')
-TOOLCHAIN_CLANG := $(ROOT_DIR)/toolchains/$(UNNAME_S)-x86/clang/clang-r383902/
+TOOLCHAIN_CLANG := $(ROOT_DIR)/toolchains/$(UNNAME_S)-x86/clang/clang-r407598b/
 TOOLCHAIN_GCC_49 := $(ROOT_DIR)/toolchains/$(UNNAME_S)-x86/gcc/aarch64-linux-gnu-4.9/
 TOOLCHAIN_GCC_51 := $(ROOT_DIR)/toolchains/$(UNNAME_S)-x86/gcc/aarch64-linux-gnu-5.1/
 TOOLCHAIN_GCC_92 := $(ROOT_DIR)/toolchains/$(UNNAME_S)-x86/gcc/aarch64-linux-gnu-9.2/