Features: Record initializer list support for Intel 14 and above

Features recorded by commit v3.6.0-rc1~120^2~5 (Features: Record
standards and features for Intel C++ on UNIX, 2016-04-28) for the Intel
compiler left out initializer list support because our test case in
`Tests/CompileFeatures/cxx_generalized_initializers.cpp` caused an
internal compiler error.  It turns out this is because the Intel
compiler asserts the `initializer_list` constructor signatures to verify
that they match its own `<initializer_list>` header.  It was our dummy
implementation used to test the language feature without any headers
that caused the ICE.  Revise it to use a constructor signature accepted
by the Intel compiler.

Fixes: #17829

Modules/Compiler/Intel-CXX-FeatureTests.cmake

@@ -31,11 +31,6 @@ set(_cmake_feature_test_cxx_aggregate_default_initializers "${Intel16_CXX14}")
 set(_cmake_feature_test_cxx_contextual_conversions "${Intel16_CXX14}")
 set(_cmake_feature_test_cxx_generic_lambdas "__cpp_generic_lambdas >= 201304")
 set(_cmake_feature_test_cxx_digit_separators "${Intel16_CXX14}")
-# This test is supposed to work in Intel 14 but the compiler has a bug
-# in versions 14 and 15::
-# https://software.intel.com/en-us/forums/intel-c-compiler/topic/600514
-# It also appears to fail with an internal compiler error on Intel 16 and 17.
-#set(_cmake_feature_test_cxx_generalized_initializers "${Intel16_CXX14}")
 unset(Intel16_CXX14)
 
 set(Intel15 "__INTEL_COMPILER >= 1500")
@@ -75,6 +70,7 @@ set(_cmake_feature_test_cxx_override "${Intel14_CXX11}")
 set(_cmake_feature_test_cxx_final "${Intel14_CXX11}")
 set(_cmake_feature_test_cxx_noexcept "${Intel14_CXX11}")
 set(_cmake_feature_test_cxx_defaulted_move_initializers "${Intel14_CXX11}")
+set(_cmake_feature_test_cxx_generalized_initializers "${Intel14_CXX11}")
 unset(Intel14_CXX11)
 
 set(Intel13_CXX11 "__INTEL_COMPILER >= 1300 && ${DETECT_CXX11}")

Tests/CompileFeatures/cxx_generalized_initializers.cpp

@@ -11,11 +11,17 @@ class initializer_list
   const _E* __begin_;
   size_t __size_;
 
+#ifdef __INTEL_COMPILER
+  // The Intel compiler internally asserts the constructor overloads, so
+  // reproduce the constructor used in its <initializer_list> header.
+  initializer_list(const _E*, size_t) {}
+#else
 public:
   template <typename T1, typename T2>
   initializer_list(T1, T2)
   {
   }
+#endif
 };
 }