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/librustc_llvm/lib.rs

@@ -1771,7 +1771,7 @@ extern {
                          Dialect: c_uint)
                          -> ValueRef;
 
-    pub static LLVMRustDebugMetadataVersion: u32;
+    pub fn LLVMRustDebugMetadataVersion() -> u32;
 
     pub fn LLVMRustAddModuleFlag(M: ModuleRef,
                                  name: *const c_char,

src/librustc_trans/trans/debuginfo/mod.rs

@@ -193,7 +193,7 @@ pub fn finalize(cx: &CrateContext) {
         // Prevent bitcode readers from deleting the debug info.
         let ptr = "Debug Info Version\0".as_ptr();
         llvm::LLVMRustAddModuleFlag(cx.llmod(), ptr as *const _,
-                                    llvm::LLVMRustDebugMetadataVersion);
+                                    llvm::LLVMRustDebugMetadataVersion());
     };
 }
 

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,