@cImport works with --cache on

We pass -MD -MF args to clang when doing `@cImport`, which
gives us a complete list of files that the C code read from.
Then we add these to the cache. So even when using `@cImport`
Zig's caching system remains perfect. This is a proof of concept
for the mechanism that the self-hosted compiler will use to
watch and rebuild files.

src/error.cpp

@@ -36,6 +36,7 @@ const char *err_str(Error err) {
         case ErrorCacheUnavailable: return "cache unavailable";
         case ErrorPathTooLong: return "path too long";
         case ErrorCCompilerCannotFindFile: return "C compiler cannot find file";
+        case ErrorReadingDepFile: return "failed to read .d file";
     }
     return "(invalid error)";
 }