Asymmetric behavior for case-insensitive matches

Case-insensitive matching with Unicode category `\p{Ll}` (lowercase letters) does not include uppercase equivalents.

```
// (?i)\p{Lu} correctly matches both cases:
RE2.compile("(?i)\\p{Lu}").matches("A") // true
RE2.compile("(?i)\\p{Lu}").matches("a") // true
// (?i)\p{Ll} only matches lowercase — the (?i) flag has no effect:
RE2.compile("(?i)\\p{Ll}").matches("a") // true
RE2.compile("(?i)\\p{Ll}").matches("A") // false — expected true
```
The same applies to `\p{Lt}` (titlecase) which is missing uppercase equivalents. Affects all scripts (Latin, Cyrillic, Greek, Armenian, etc.).