Compiler.h: Disable initializer list usage with clang-cl

Most people are using MSVC 2012, which lacks the <initializer_list>
header.  MSVC 2013 shipped with that header, but it has not yet been
tested.  If clang works with the 2013 header, then we can enable this by
checking the value of _MSC_VER.

llvm-svn: 196448