CodeGen support for global variables of type std::initializer_list<X>.

CodeGen support for global variables of type std::initializer_list<X>.

This emits a backing array with internal linkage and fills it with data,
then has the initializer_list point at the array. Dynamic initialization
and global destructors are correctly supported.

What doesn't work is nested initializer_lists. I have no idea how to
get them to work, either. However, these should be very rare, and so
I'll just call it a known bug and declare generalized initializers
DONE!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@151457 91177308-0d34-0410-b5e6-96231b3b80d8