Add LLVM-pass plugin support to LDC. Commandline flag: -plugin=.... (#2554)

This adds functionality to load plugins such as the AFL llvm-mode plugin: https://github.com/mirrorer/afl/blob/master/llvm_mode/afl-llvm-pass.so.cc
Note that such plugins developed for Clang should also work for LDC !

tests/plugins/addFuncEntryCall/addFuncEntryCallPass.cpp

@@ -0,0 +1,60 @@
+//===-- addFuncEntryCallPass.cpp - Optimize druntime calls ----------------===//
+//
+//                         LDC – the LLVM D compiler
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See the LICENSE file for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Pass.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/IRBuilder.h"
+#include "llvm/IR/LegacyPassManager.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Transforms/IPO/PassManagerBuilder.h"
+
+using namespace llvm;
+
+namespace {
+
+class FuncEntryCallPass : public FunctionPass {
+
+  Constant *funcToCallUponEntry = nullptr;
+
+public:
+  static char ID;
+  FuncEntryCallPass() : FunctionPass(ID) {}
+
+  bool doInitialization(Module &M) override;
+  bool runOnFunction(Function &F) override;
+};
+}
+
+char FuncEntryCallPass::ID = 0;
+
+bool FuncEntryCallPass::doInitialization(Module &M) {
+  // Add fwd declaration of the `void __test_funcentrycall(void)` function.
+  auto functionType = FunctionType::get(Type::getVoidTy(M.getContext()), false);
+  funcToCallUponEntry =
+      M.getOrInsertFunction("__test_funcentrycall", functionType);
+  return true;
+}
+
+bool FuncEntryCallPass::runOnFunction(Function &F) {
+  // Add call to `__test_funcentrycall(void)` at the start of _every_ function
+  // (this includes e.g. `ldc.register_dso`!)
+  llvm::BasicBlock &block = F.getEntryBlock();
+  IRBuilder<> builder(&block, block.begin());
+  builder.CreateCall(funcToCallUponEntry);
+  return true;
+}
+
+static void addFuncEntryCallPass(const PassManagerBuilder &,
+                                 legacy::PassManagerBase &PM) {
+  PM.add(new FuncEntryCallPass());
+}
+// Registration of the plugin's pass is done by the plugin's static constructor.
+static RegisterStandardPasses
+    RegisterFuncEntryCallPass0(PassManagerBuilder::EP_EnabledOnOptLevel0,
+                               addFuncEntryCallPass);