[AnnotationRemarks] Use subprogram location for summary remarks.

[AnnotationRemarks] Use subprogram location for summary remarks.

The summary remarks are generated on a per-function basis. Using the
first instruction's location is sub-optimal for 2 reasons:
  1. Sometimes the first instruction is missing !dbg
  2. The location of the first instruction may be mis-leading.

Instead, just use the location of the function directly.