rustc_llvm: Don't export constants across dlls

For imports of constants across DLLs to work on Windows it *requires* that the
import be marked with `dllimport` (unlike functions where the marker is
optional, but strongly recommended). This currently isn't working for importing
FFI constants across boundaries, however, so the one constant exported from
`rustc_llvm.dll` is now a function to be called instead.

src/rustllvm/RustWrapper.cpp

@@ -233,7 +233,9 @@ DIT unwrapDI(LLVMMetadataRef ref) {
     return DIT(ref ? unwrap<MDNode>(ref) : NULL);
 }
 
-extern "C" const uint32_t LLVMRustDebugMetadataVersion = DEBUG_METADATA_VERSION;
+extern "C" const uint32_t LLVMRustDebugMetadataVersion() {
+    return DEBUG_METADATA_VERSION;
+}
 
 extern "C" void LLVMRustAddModuleFlag(LLVMModuleRef M,
                                       const char *name,