I don't think those results really support the point that you're trying to make.
The top result, which is the most significant, is pretty much irrelevant, for example. It's limited to Haskell-related packages. Most Debian users have absolutely no reason to use those packages.
After that, we start running into rather obscure options that only GCC supports, or bad code that GCC accepts, or code that's just bad in general, or extremely obscure packages that very few people would ever consider using. These don't indicate a problem with LLVM or Clang.
The fact that FreeBSD managed to very successfully switch to Clang for compiling the kernel and base system, and the fact that it's very widely used on OS X, suggests to me that LLVM and Clang are more than robust enough for everyday use.
I wouldn't use “robust” here. Just look at the errors types and the examples (below the table).
Yes, there are some features that Clang doesn't implement at the moment or never will (GCC has some really bizarre extensions and is sometimes less strict), but there is also a great amount caused by bad code.
I can't come up with a better alternative, though.
