Code generation support for C99 designated initializers.

The approach I've taken in this patch is relatively straightforward,
although the code itself is non-trivial. Essentially, as we process
an initializer list we build up a fully-explicit representation of the
initializer list, where each of the subobject initializations occurs
in order. Designators serve to "fill in" subobject initializations in
a non-linear way. The fully-explicit representation makes initializer
lists (both with and without designators) easy to grok for codegen and
later semantic analyses. We keep the syntactic form of the initializer
list linked into the AST for those clients interested in exactly what
the user wrote.

Known limitations:
  - Designating a member of a union that isn't the first member may
    result in bogus initialization (we warn about this)
  - GNU array-range designators are not supported (we warn about this)

llvm-svn: 63242