Avoid non-attributive uses of 'unsupported' in diagnostics

We don't have a style guide for diagnostic messages, but convention strongly
favours the forms:

  'attribute is not supported', 'unsupported attribute'

We generally avoid:

  'attribute is unsupported', 'non-supported attribute'

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212972 91177308-0d34-0410-b5e6-96231b3b80d8