Use name-discarding LLVM context

This is only applicable when neither of --emit=llvm-ir or --emit=llvm-bc are not
requested.

In case either of these outputs are wanted, but the benefits of such context are
desired as well, -Zfewer_names option provides the same functionality regardless
of the outputs requested.

src/rustllvm/rustllvm.h

@@ -71,7 +71,7 @@
 #include "llvm/IR/IRPrintingPasses.h"
 #include "llvm/Linker/Linker.h"
 
-void LLVMRustSetLastError(const char *);
+extern "C" void LLVMRustSetLastError(const char *);
 
 enum class LLVMRustResult { Success, Failure };