sfinae-protect operator== StringRef overload

[vhavel]

We encountered a c++ compilation issue when using p4c together with inja (https://github.com/pantor/inja) in one project. Inja headers define:

  template <class CharT, class Traits nssv_MSVC_ORDER(1)>
  nssv_constexpr bool operator==(basic_string_view<CharT, Traits> lhs,
                                 nssv_BASIC_STRING_VIEW_I(CharT, Traits) rhs) nssv_noexcept {

which is supposed to be called when basic_string_view<> is compared to const char*. But if p4c sources are also included, such call is ambiguous, because operator==(T, StringRef) also participates in overload resolution:

inja/include/inja/parser.hpp:207:31: error: ambiguous overload for ‘operator==’ (operand types are ‘nonstd::sv_lite::string_view {aka nonstd::sv_lite::basic_string_view<char>}’ and ‘const char [3]’)
           } else if (tok.text == "in") {
                      ~~~~~~~~~^~~~~~~

p4c/lib/stringref.h:145:10: note: candidate: bool operator==(T, const StringRef&) [with T = nonstd::sv_lite::basic_string_view<char>]
     bool operator==(T a, const StringRef &b) { return b == a; }
          ^~~~~~~~

inja/include/inja/string_view.hpp:1088:18: note: candidate: constexpr bool nonstd::sv_lite::operator==(nonstd::sv_lite::basic_string_view<CharT, Traits>, typename std::decay<nonstd::sv_lite::basic_string_view<CharT, Traits> >::type) [with CharT = char; Traits = std::char_traits<char>; typename std::decay<nonstd::sv_lite::basic_string_view<CharT, Traits> >::type = nonstd::sv_lite::basic_string_view<char>]
   nssv_constexpr bool operator==(basic_string_view<CharT, Traits> lhs,
                  ^~~~~~~~

This PR fixes this problem by employing SFINAE to enable operator==(T a, StringRef b) only if a == b is a valid expression.

[mihaibudiu]

Frankly I don't know enough C++ to judge whether this is an improvement, but since the tests pass I will mark this as accepted. Ideally @ChrisDodd should review this, but he's been busy lately.