[llvm-c-test] Fix a warning

This patch fixes:

  llvm/tools/llvm-c-test/debuginfo.c:447:27: error: unused variable
  'ME' [-Werror,-Wunused-variable]

Author: kazutakahirata

llvm/tools/llvm-c-test/debuginfo.c

@@ -446,6 +446,7 @@ int llvm_add_globaldebuginfo(void) {
   size_t numEntries;
   LLVMValueMetadataEntry *ME = LLVMGlobalCopyAllMetadata(Global, &numEntries);
   assert(ME != NULL);
+  (void)ME;
   assert(numEntries == 2);
 
   LLVMDisposeDIBuilder(Builder);