A number of array-related IR-gen cleanups.

  - Emit default-initialization of arrays that were partially initialized
    with initializer lists with a loop, rather than emitting the default
    initializer N times;
  - support destroying VLAs of non-trivial type, although this is not
    yet exposed to users; and
  - support the partial destruction of arrays initialized with
    initializer lists when an initializer throws an exception.

llvm-svn: 134784