Make llvmc work again.

Chris recently broke llvmc with his Makefile changes (r75379). That patch made
the global change .o -> .a, which caused built-in llvmc plugins to stop working
since plugin initialization in llvmc is based on static variables not referenced
from the main executable. This patch implements auto-generated forced references
to the plugin libraries.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@74000 91177308-0d34-0410-b5e6-96231b3b80d8

utils/TableGen/LLVMCConfigurationEmitter.cpp

@@ -1984,6 +1984,7 @@ void EmitRegisterPlugin(int Priority, std::ostream& O) {
 /// additional declarations.
 void EmitIncludes(std::ostream& O) {
   O << "#include \"llvm/CompilerDriver/CompilationGraph.h\"\n"
+    << "#include \"llvm/CompilerDriver/ForceLinkageMacros.h\"\n"
     << "#include \"llvm/CompilerDriver/Plugin.h\"\n"
     << "#include \"llvm/CompilerDriver/Tool.h\"\n\n"
 
@@ -2106,7 +2107,13 @@ void EmitPluginCode(const PluginData& Data, std::ostream& O) {
   // Emit code for plugin registration.
   EmitRegisterPlugin(Data.Priority, O);
 
-  O << "} // End anonymous namespace.\n";
+  O << "} // End anonymous namespace.\n\n";
+
+  // Force linkage magic.
+  O << "namespace llvmc {\n";
+  O << "LLVMC_FORCE_LINKAGE_DECL(LLVMC_PLUGIN_NAME) {}\n";
+  O << "}\n";
+
   // EOF
 }