Calling a function with a struct that has a tainted field should taint every field

[mlevesquedion]

The following test cases were introduced by #195 and are currently failing:

func TestCallWithStructReferenceTaintsEveryField(h Headers) {
	fooByPtr(&h)       // without interprocedural assessment, foo can do anything, so this call should taint every field on h
	core.Sink(h.Name)  // TODO want "a source has reached a sink"
	core.Sink(h.Other) // TODO want "a source has reached a sink"
}

func TestCallWithStructValueDoesNotTaintNonReferenceFields(h Headers) {
	foo(h) // h is passed by value, so only its reference-like fields should be tainted
	core.Sink(h.Name)
	core.Sink(h.Other) // TODO want "a source has reached a sink"
}

type Headers struct {
	Name  string
	Auth  map[string]string `levee:"source"`
	Other map[string]string
}

func fooByPtr(h *Headers) {}

func foo(h Headers) {}

The crux of the issue here is that since one of Headers's fields is a source, that source will be in scope inside the caller and so in theory it could taint every other field. Without interprocedural analysis, we have to assume that every field will be tainted.

In fact, we have to do this inference whenever a struct with a tainted field is passed to a call, not just when a field is a source.

See #195 for additional discussion.