[NFC][libc++][exceptions] Adds tests for LWG3112.

The tests kept being based on std::string instead of std::string_view to allow
testing with older C++ dialects.

Adds tests for:
- LWG3112 system_error and filesystem_error constructors taking a string may not be able to meet their postconditions

Author: mordante

libcxx/docs/Status/Cxx20Issues.csv

@@ -134,7 +134,7 @@
 "`3077 <https://wg21.link/LWG3077>`__","``(push|emplace)_back``\  should invalidate the ``end``\  iterator","Kona","|Nothing To Do|",""
 "`3087 <https://wg21.link/LWG3087>`__","One final ``&x``\  in |sect|\ [list.ops]","Kona","|Nothing To Do|",""
 "`3101 <https://wg21.link/LWG3101>`__","``span``\ 's ``Container``\  constructors need another constraint","Kona","|Complete|",""
-"`3112 <https://wg21.link/LWG3112>`__","``system_error``\  and ``filesystem_error``\  constructors taking a ``string``\  may not be able to meet their postconditions","Kona","",""
+"`3112 <https://wg21.link/LWG3112>`__","``system_error``\  and ``filesystem_error``\  constructors taking a ``string``\  may not be able to meet their postconditions","Kona","|Nothing To Do|",""
 "`3119 <https://wg21.link/LWG3119>`__","Program-definedness of closure types","Kona","|Nothing To Do|",""
 "`3133 <https://wg21.link/LWG3133>`__","Modernizing numeric type requirements","Kona","",""
 "`3144 <https://wg21.link/LWG3144>`__","``span``\  does not have a ``const_pointer``\  typedef","Kona","|Complete|",""

libcxx/test/std/diagnostics/syserr/syserr.syserr/syserr.syserr.members/ctor_error_code_string.pass.cpp

@@ -21,11 +21,12 @@
 #include "test_macros.h"
 
 int main(int, char**) {
-    std::string what_arg("test message");
+    std::string what_arg("test message\0With embedded NUL");
     std::system_error se(make_error_code(std::errc::not_a_directory), what_arg);
     assert(se.code() == std::make_error_code(std::errc::not_a_directory));
     std::string what_message(se.what());
     assert(what_message.find(what_arg) != std::string::npos);
+    assert(what_message.find(what_arg.c_str()) != std::string::npos);
     assert(what_message.find("Not a directory") != std::string::npos);
 
     return 0;

libcxx/test/std/diagnostics/syserr/syserr.syserr/syserr.syserr.members/ctor_int_error_category_string.pass.cpp

@@ -21,12 +21,13 @@
 #include "test_macros.h"
 
 int main(int, char**) {
-    std::string what_arg("test message");
+    std::string what_arg("test message\0With embedded NUL");
     std::system_error se(static_cast<int>(std::errc::not_a_directory),
                          std::generic_category(), what_arg);
     assert(se.code() == std::make_error_code(std::errc::not_a_directory));
     std::string what_message(se.what());
     assert(what_message.find(what_arg) != std::string::npos);
+    assert(what_message.find(what_arg.c_str()) != std::string::npos);
     assert(what_message.find("Not a directory") != std::string::npos);
 
     return 0;

libcxx/test/std/input.output/filesystems/class.filesystem_error/filesystem_error.members.pass.cpp

@@ -33,13 +33,13 @@ namespace fs = std::filesystem;
 void test_constructors() {
   using namespace fs;
 
-  // The string returned by "filesystem_error::what() must contain runtime_error::what()
-  const std::string what_arg = "Hello World";
+  // The string returned by "filesystem_error::what() must contain runtime_error::what().c_str()
+  const std::string what_arg = "Hello World\0with embedded NUL";
   const std::string what_contains = std::runtime_error(what_arg).what();
   assert(what_contains.find(what_arg) != std::string::npos);
   auto CheckWhat = [what_contains](filesystem_error const& e) {
     std::string s = e.what();
-    assert(s.find(what_contains) != std::string::npos);
+    assert(s.find(what_contains.c_str()) != std::string::npos);
   };
 
   std::error_code ec = std::make_error_code(std::errc::file_exists);