[-Wunsafe-buffer-usage] Check missing and duplicated assignments

[patrykstefanski]

Add checks for missing and duplicated assignments in bounds-attributed groups. This model requires each mutable decl in a group to be assigned exactly once.

rdar://161608243

[ziqingluo-90]

LGTM, thank you. Just have one nitpick.

Btw, is checking dependencies between assignments within one assignment group something we planned to do? For example,

void f(int * __counted_by(n) ptr, size_t n) {
  ptr = span.first(n).data();
  n = 42;  // this assignment should happen first
}

[ziqingluo-90]

Another option could be using notes to list these objects that need to be assigned exactly once. Each Decl per note.
This way, we could make the warning message shorter and less parameterized (this benefits grep-ing warning messages), and provide more information in notes, e.g., adding " ... need to be assigned exactly once." and pointing to their source locations.