[dfsan] Add a flag about whether to propagate offset labels at gep

DFSan has flags to control flows between pointers and objects referred
by pointers. For example,

a = *p;
L(a) = L(*p)        when -dfsan-combine-pointer-labels-on-load = false
L(a) = L(*p) + L(p) when -dfsan-combine-pointer-labels-on-load = true

*p = b;
L(*p) = L(b)        when -dfsan-combine-pointer-labels-on-store = false
L(*p) = L(b) + L(p) when -dfsan-combine-pointer-labels-on-store = true
The question is what to do with p += c.

In practice we found many confusing flows if we propagate labels from c
to p. So a new flag works like this

p += c;
L(p) = L(p)        when -dfsan-propagate-via-pointer-arithmetic = false
L(p) = L(p) + L(c) when -dfsan-propagate-via-pointer-arithmetic = true

Reviewed-by: gbalats

Differential Revision: https://reviews.llvm.org/D103176