Support/ConvertUTF: implement U+FFFD insertion according to the recommendation
given in the Unicode spec

That is, replace every maximal subpart of an ill-formed subsequence with one
U+FFFD.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211015 91177308-0d34-0410-b5e6-96231b3b80d8