adopt clang's diagnostics to beluga

[mycoboco]

• [ ] inconsistent printf arguments
• [x] range highlight
• ~~"fix it" - correct common mistakes~~
• [x] display lines before and after macro expansion (#3)
• [x] missing terminating semicolon should be diagnosed at the end of line rather than at the start of next line (#34)
• [ ] warn explicit out-of-bound; int a[100]; return a[100];
• [ ] warn memset(p, 0, sizeof(p))
• [x] warn using bit ops in comparison without parentheses (#51)
• [x] warn mixing && and || without parentheses (#49)
• [x] warn if (a = b) but not if ((a = b)) (#50)
• [x] warn 42 + b? x: y without parentheses (#53; decided not to implement)
• [x] warn empty body of if (#47)
• [x] diagnose foo_t P = 42; as unknown type (#54)
• [x] better diagnostics for (undecled_id) taking it as cast (#55)

[mycoboco]

Need to consider issuing a warning for 42 + b? x: y or b? x: y + 42 is really worthwhile.

[mycoboco]

Decided not to support the "fix it" feature.