Make LDC work with LLVM trunk (s/LinkOnceLinkage/LinkOnceOdrLinkage/)

Also moved the #defines for linkage types into a separate header instead of
mars.h so we can #include revisions.h without having to rebuild the entire
frontend every time we update.
(I'm using revisions.h to get the LLVM revision for use in preprocessor
conditionals. It should work with LLVM release 2.5, old trunk and new trunk)

gen/todebug.cpp

@@ -12,6 +12,7 @@
 #include "gen/tollvm.h"
 #include "gen/logger.h"
 #include "gen/llvmhelpers.h"
+#include "gen/linkage.h"
 
 #include "ir/irmodule.h"
 
@@ -33,7 +34,9 @@ using namespace llvm::dwarf;
  */
 static LLGlobalVariable* emitDwarfGlobalDecl(const LLStructType* type, const char* name, bool linkonce=false)
 {
-    LLGlobalValue::LinkageTypes linkage = linkonce ? LLGlobalValue::LinkOnceLinkage : LLGlobalValue::InternalLinkage;
+    LLGlobalValue::LinkageTypes linkage = linkonce
+        ? DEBUGINFO_LINKONCE_LINKAGE_TYPE
+        : LLGlobalValue::InternalLinkage;
     LLGlobalVariable* gv = new LLGlobalVariable(type, true, linkage, NULL, name, gIR->module);
     gv->setSection("llvm.metadata");
     return gv;