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