Diagnose invalid <LANG>_STANDARD value instead of crashing (#15426)

Source/cmLocalGenerator.cxx

@@ -2270,7 +2270,14 @@ AddCompilerRequirementFlag(std::string &flags, cmTarget* target,
 
   std::vector<std::string>::const_iterator stdIt =
                                 std::find(stds.begin(), stds.end(), standard);
-  assert(stdIt != stds.end());
+  if (stdIt == stds.end())
+    {
+    std::string e =
+      lang + "_STANDARD is set to invalid value '" + standard + "'";
+    this->GetGlobalGenerator()->GetCMakeInstance()
+      ->IssueMessage(cmake::FATAL_ERROR, e, target->GetBacktrace());
+    return;
+    }
 
   std::vector<std::string>::const_iterator defaultStdIt =
     std::find(stds.begin(), stds.end(), defaultStd);

Tests/RunCMake/CompileFeatures/NotAStandard-result.txt

@@ -0,0 +1 @@
+1

Tests/RunCMake/CompileFeatures/NotAStandard-stderr.txt

@@ -0,0 +1,4 @@
+CMake Error at NotAStandard.cmake:[0-9]+ \(add_library\):
+  CXX_STANDARD is set to invalid value 'bad'
+Call Stack \(most recent call first\):
+  CMakeLists.txt:[0-9]+ \(include\)

Tests/RunCMake/CompileFeatures/NotAStandard.cmake

@@ -0,0 +1,2 @@
+set(CMAKE_CXX_STANDARD bad)
+add_library(somelib STATIC empty.cpp)

Tests/RunCMake/CompileFeatures/RunCMakeTest.cmake

@@ -43,6 +43,8 @@ else()
 endif()
 
 if(CXX_STANDARD_DEFAULT)
+  run_cmake(NotAStandard)
+
   foreach(standard 98 11)
     file(READ
       "${RunCMake_BINARY_DIR}/generate_feature_list-build/cxx${standard}_flag.txt"