[Assignment Tracking] Only set module flag if pass modifies a function

The module flag "debug-info-assignment-tracking" is set by the
declare-to-assign pass to indicate that assignment tracking is enabled. This
patch changes declare-to-assign to only set the flag if it makes a
modification. This lets the compiler avoid doing extra work for no benefit,
such as is currently the case if assignment tracking is requested for a build
with line tables only (-gmlt) or a build without debug info.

Reviewed By: scott.linder

Differential Revision: https://reviews.llvm.org/D144481