Use C type when passing value to LLVM pass

Previously the C type LLVMRelocMode (available as RelocMode in Rust)
was passed as is to the function.
However createTargetMachine expects a Reloc::Model, which is an enum
just one value short.
Additionally, the function was marked as requiring Reloc::Model in the
C code, but RelocMode on the Rust-side.

We now use the correct C type LLVMRelocMode and convert it to an
Optional<Reloc::Model> as expected by the createTargetMachine call the
same the original LLVMCreateTargetMachine function does.
See
https://github.com/llvm-mirror/llvm/blob/c9b262bfbd5b9fb6f10749dba1465569f39bd625/lib/Target/TargetMachineC.cpp#L104-L121

This was found by @eddyb.

src/rustllvm/PassWrapper.cpp

@@ -167,12 +167,35 @@ LLVMRustCreateTargetMachine(const char *triple,
                             const char *cpu,
                             const char *feature,
                             CodeModel::Model CM,
-                            Reloc::Model RM,
+                            LLVMRelocMode Reloc,
                             CodeGenOpt::Level OptLevel,
                             bool UseSoftFloat,
                             bool PositionIndependentExecutable,
                             bool FunctionSections,
                             bool DataSections) {
+
+#if LLVM_VERSION_MINOR <= 8
+    Reloc::Model RM;
+#else
+    Optional<Reloc::Model> RM;
+#endif
+    switch (Reloc){
+        case LLVMRelocStatic:
+            RM = Reloc::Static;
+            break;
+        case LLVMRelocPIC:
+            RM = Reloc::PIC_;
+            break;
+        case LLVMRelocDynamicNoPic:
+            RM = Reloc::DynamicNoPIC;
+            break;
+        default:
+#if LLVM_VERSION_MINOR <= 8
+            RM = Reloc::Default;
+#endif
+            break;
+    }
+
     std::string Error;
     Triple Trip(Triple::normalize(triple));
     const llvm::Target *TheTarget = TargetRegistry::lookupTarget(Trip.getTriple(),