[libcxx] [test] D27021: Fix MSVC warning C4389 "signed/unsigned mismatch", part 8/12.

Add static_cast<std::size_t> when comparing distance() to size().

These replacements were performed programmatically with regex_replace():

const vector<pair<regex, string>> reg_fmt = {
    { regex(R"(assert\((\w+)\.size\(\) == std::distance\((\w+, \w+)\)\))"),
        "assert($1.size() == static_cast<std::size_t>(std::distance($2)))" },
    { regex(R"(assert\(distance\((\w+\.begin\(\), \w+\.end\(\))\) == (\w+)\.size\(\)\))"),
        "assert(static_cast<std::size_t>(distance($1)) == $2.size())" },
    { regex(R"(assert\(std::distance\((\w+\.\w*begin\(\), \w+\.\w*end\(\))\) == (\w+)\.size\(\)\))"),
        "assert(static_cast<std::size_t>(std::distance($1)) == $2.size())" },
};

Also, include <cstddef> when it wasn't already being included.

llvm-svn: 288745