Provide anchor for compiler extensions

This patch is cherry-picked from 04b0a4e22e3b4549f9d241f8a9f37eebecb62a31, and
amended to prevent an undefined reference to `llvm::EnableABIBreakingChecks'

lib/Extensions/Extensions.cpp

@@ -0,0 +1,15 @@
+#include "llvm/Passes/PassPlugin.h"
+#define HANDLE_EXTENSION(Ext)                                                  \
+		llvm::PassPluginLibraryInfo get##Ext##PluginInfo();
+#include "llvm/Support/Extension.def"
+
+
+namespace llvm {
+	namespace details {
+		void extensions_anchor() {
+#define HANDLE_EXTENSION(Ext)                                                  \
+			static auto Ext = get##Ext##PluginInfo();
+#include "llvm/Support/Extension.def"
+		}
+	}
+}

lib/Extensions/LLVMBuild.txt

@@ -18,4 +18,4 @@
 type = Library
 name = Extensions
 parent = Libraries
-required_libraries =
+required_libraries = Support