[Diagnostics] Diagnose missing comma in string array initialization

Motivation (from PR37674):

const char *ss[] = {
  "foo", "bar",
  "baz", "qux"  // <-- Missing comma!
  "abc", "xyz"
  };

This kind of bug was recently also found in LLVM codebase (see PR47030).

Solves PR47038, PR37674

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D85545