Ninja: Restore support for compilers not defining a C++ standard level

Since commit 386465bf83 (cmTarget: add support for C++ module fileset
types, 2022-04-08, v3.25.0-rc1~624^2~7), the Ninja generator checks for
C++20 support using logic that requires `CMAKE_<LANG>_STANDARD_DEFAULT`
to be non-empty.  On some compilers, such as ARMClang, CMake does not
automatically detect and set default language standards, thus causing
`HaveStandardAvailable` to raise an internal error.

To fix this issue, if `CMAKE_CXX_STANDARD_DEFAULT` is empty, assume all
standards to be supported instead of calling `HaveStandardAvailable`.
This is consistent with how `CompileFeaturesNode::Evaluate` handles this
case.

Fixes: #24146

Source/cmGeneratorTarget.cxx

@@ -8798,11 +8798,18 @@ cmGeneratorTarget::Cxx20SupportLevel cmGeneratorTarget::HaveCxxModuleSupport(
   if (!state->GetLanguageEnabled("CXX")) {
     return Cxx20SupportLevel::MissingCxx;
   }
-  cmStandardLevelResolver standardResolver(this->Makefile);
-  if (!standardResolver.HaveStandardAvailable(this, "CXX", config,
-                                              "cxx_std_20")) {
-    return Cxx20SupportLevel::NoCxx20;
-  }
+  cmValue standardDefault =
+    this->Target->GetMakefile()->GetDefinition("CMAKE_CXX_STANDARD_DEFAULT");
+  if (standardDefault && !standardDefault->empty()) {
+    cmStandardLevelResolver standardResolver(this->Makefile);
+    if (!standardResolver.HaveStandardAvailable(this, "CXX", config,
+                                                "cxx_std_20")) {
+      return Cxx20SupportLevel::NoCxx20;
+    }
+  }
+  // Else, an empty CMAKE_CXX_STANDARD_DEFAULT means CMake does not detect and
+  // set a default standard level for this compiler, so assume all standards
+  // are available.
   if (!this->Makefile->IsOn("CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP")) {
     return Cxx20SupportLevel::MissingExperimentalFlag;
   }