sanmd: improve precision of UAR analysis

Only mark functions that have address-taken locals
as requiring UAR checking.

On a large internal app this reduces number of marked functions
from 78441 to 66618. Mostly small, trivial getter/setter-type
functions are unmarked, but also some amount of larger
number-crunching-type functions are unmarked as well.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D139811