Fix assertion "Chunk.Kind == DeclaratorChunk::Function" with attributed type.

This patch is to upgrade FunctionTypeUnwrapper for correct processing of
AttributedType. Fixes PR25786.
Patch by Alexander Makarov.

Differential Revision: http://reviews.llvm.org/D15373


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@260373 91177308-0d34-0410-b5e6-96231b3b80d8