Sema: handle AttributedTypeLocs in C++14 auto deduction

When performing a type deduction from the return type, the FunctionDecl may be
attributed with a calling convention.  In such a case, the retrieved type
location may be an AttributedTypeLoc.  Performing a castAs<FunctionProtoTypeLoc>
on such a type loc would result in an assertion as they are not derived types.

Ensure that we correctly handle the attributed type location by looking through
it to the modified type loc.

Fixes an assertion triggered in C++14 mode.

llvm-svn: 219851