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

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
 };
 }