Fix non-MSVC build error in ASTContext::getAdjustedType

Use FunctionTypeUnwrapper like we do in AttributedType to try to keep
some sugar.  We can actually do one better here in the future by
avoiding the AdjustedType node altogether when no sugar would be lost.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196455 91177308-0d34-0410-b5e6-96231b3b80d8