[libc++] Improve make_string test support.

[Mordante]

Summary

Adds MAKE_CSTRING and makes the operators of MultiStringType constexpr.

The code is copied from D96664 so it can be used in D80895.

[• Quuxplusone]

Sure; although here are some non-blockers:

• the repeated comment is maybe overkill
• the existing code's use of implicit conversion operators (instead of say an .as_chars<CharT>() getter) is just horrible and I'd love to see a PR to fix it throughout the test suite
• I wonder if you're going to need MAKE_STRING_VIEW next

[curdeius]

LGTM. I second what Arthur said.

[Mordante]

Thanks for the reviews. I'll update the comment before landing.
I did quite a bit more on std::format than what's up for review. I haven't had the need for MAKE_STRING_VIEW so I don't foresee I'll ever need it :-)
I think you've a good point about the implicit conversion. I'll add it to my todo list.

libcxx/test/support/make_string.h

struct MultiStringType {
   const char* s;
   const wchar_t* w;
 #if TEST_STD_VER > 17 && defined(__cpp_char8_t)
   const char8_t* u8;
 #endif
   const char16_t* u16;
   const char32_t* u32;
 
-  operator const char*() const { return s; }
-  operator const wchar_t*() const { return w; }
+  constexpr operator const char*() const { return s; }
+  constexpr operator const wchar_t*() const { return w; }
 #if TEST_STD_VER > 17 && defined(__cpp_char8_t)
-  operator const char8_t*() const { return u8; }
+  constexpr operator const char8_t*() const { return u8; }
 #endif
-  operator const char16_t*() const { return u16; }
-  operator const char32_t*() const { return u32; }
+  constexpr operator const char16_t*() const { return u16; }
+  constexpr operator const char32_t*() const { return u32; }
 };
 
 // Helper to convert a const char* string to a basic_string<CharT>.
 // This helper is used in unit tests to make them generic. The input should be
 // valid ASCII which means the input is also valid UTF-8.
 #define MAKE_STRING(CharT, Str)                                                \
   std::basic_string<CharT> {                                                   \
     static_cast<const CharT*>(MultiStringType MKSTR(Str))                      \
   }
 
+// Like MAKE_STRING but converts to a const CharT*.
+#define MAKE_CSTRING(CharT, Str)                                               \
+  static_cast<const CharT*>(MultiStringType MKSTR(Str))
+
 #endif