designated initializers. This implementation should cover all of the
constraints in C99 6.7.8, including long, complex designations and
computing the size of incomplete array types initialized with a
designated initializer. Please see the new test-case and holler if you
find cases where this doesn't work.

There are still some wrinkles with GNU's anonymous structs and
anonymous unions (it isn't clear how these should work; we'll just
follow GCC's lead) and with designated initializers for the members of a
union. I'll tackle those very soon.

CodeGen is still nonexistent, and there's some leftover code in the
parser's representation of designators that I'll also need to clean up.

llvm-svn: 62737