Features: Record initializer list support for Intel 14 and above

Features recorded by commit be910f00 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 (closed)