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 !

driver/plugins.cpp

@@ -0,0 +1,48 @@
+//===-- driver/plugins.cpp -------------------------------------*-  C++ -*-===//
+//
+//                         LDC – the LLVM D compiler
+//
+// This file is distributed under the BSD-style LDC license. See the LICENSE
+// file for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// Implements functionality related to plugins (`-plugin=...`).
+//
+//===----------------------------------------------------------------------===//
+
+#include "driver/plugins.h"
+
+#if LDC_ENABLE_PLUGINS
+
+#include "errors.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/DynamicLibrary.h"
+
+namespace {
+namespace cl = llvm::cl;
+
+cl::list<std::string>
+    pluginFiles("plugin", cl::CommaSeparated, cl::desc("Plugins to load."),
+                cl::value_desc("<dynamic_library.so, lib2.so>"));
+
+} // anonymous namespace
+
+/// Loads all plugins. The static constructor of each plugin should take care of
+/// the plugins registering themself with the rest of LDC/LLVM.
+void loadAllPlugins() {
+  for (auto &filename : pluginFiles) {
+    std::string errorString;
+    if (llvm::sys::DynamicLibrary::LoadLibraryPermanently(filename.c_str(),
+                                                          &errorString)) {
+      error(Loc(), "Error loading plugin '%s': %s", filename.c_str(),
+            errorString.c_str());
+    }
+  }
+}
+
+#else // #if LDC_ENABLE_PLUGINS
+
+void loadAllPlugins() {}
+
+#endif // LDC_ENABLE_PLUGINS