Implementation of gcc mode attribute; this is significant because

Implementation of gcc mode attribute; this is significant because
it fixes PR2204.  Not too much to say about the implementation; it works
in a similar way to the vector size attribute.

At some point, we need to modify the targets to provide information
about the appropriate types.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@51577 91177308-0d34-0410-b5e6-96231b3b80d8