[C99] Claim partial conformance to n448
This is the paper that added the 'restrict' keyword. Clang is
conforming to the letter of the standard's requirements, so it would be
defensible for us to claim full support instead. However, LLVM does not
currently support the optimization semantics with restricted local
variables or data members, only with restricted pointers declared in
function parameters. So we're only claiming partial support because we
don't yet take full advantage of what the feature allows.
