Correctly handle designated initializers which modify an array initialized
with a string.  This case is sort of tricky because we can't modify the
StringLiteral used to represent such initializers.
We are forced to decompose the string into individual characters.

Fixes <rdar://problem/10465114>.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@183791 91177308-0d34-0410-b5e6-96231b3b80d8