Updating the diagnostics as per the feedback on
https://reviews.llvm.org/D122627.
This change correctly handles missing argument lists, and changes the
subject for the `numthreads` attribute to be global functions.
I did not handle applying the attribute to template functions because
that currently fails parsing in a way that is consisetent with the
current DXC codebase (Microsoft attributes are not supported on
templates).
A future improvement to the diagnostic maybe warranted.
ff6696c842