[multiprecision-builtins] Added missing builtin __builtin_{add,sub}cb for {add,sub} with carry for bytes.

I have had several people ask me about why this builtin was not available in
clang (since it seems like a logical conclusion). This patch implements said
builtins.

Relevant tests are included as well. I also updated the Clang language extension reference.

rdar://14192664.