[Sema] Fix reporting of invalid shader attribute on HLSL entry function

If the HLSL entry function had a shader attribute that conflicted
with the pipeline stage specified in the target triple, Clang
would emit:

   error: (null) attribute parameters do not match the previous declaration
   conflicting attribute is here

(where the second line doesn't reference an attribute).

This was because the code constructed a dummy attribute that had
only a source location, but no kind or syntax.

Noticed while doing some changes to the attribute handling.

Differential Revision: https://reviews.llvm.org/D147657