Make ldc compatible with LLVM 3.4

Development of LLVM 3.4 started with a cleanup of the path class (PathV1).
The changes here let ldc compile at least with rev. 184039 of LLVM.

gen/todebug.cpp

@@ -27,6 +27,9 @@
 #include "llvm/Support/Dwarf.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Path.h"
+#if LDC_LLVM_VER >= 304
+#include "llvm/Support/PathV1.h"
+#endif
 
 using namespace llvm::dwarf;