handle the unify-by-value semantics in the EAR pointer analysis

[guodongli-google]

Bug report

Go's objects are copy-by-value by default, so copying a struct (or an array) actually copying its elements rather than the reference of the struct. The EAR analysis ("earpointer/analysis.go") approximates this using copy-by-reference. For example,

type T1 struct { ... }

type T2 struct {
  x T1
  y *int
}

func f(t2 T2) {
  t3 := t2
  ...
}

Currently the analysis unifies t3 and t2 to be {t2, t3}. However w.r.t. the exact semantics we should have {t3.y, t2.y}, and t2.x and t3.x should be unified recursively.
This approximation may lead to false positives.