[flang] Implement folding of INDEX, SCAN, & VERIFY

Fold LEN_TRIM

Fold REPEAT

Fix gcc build warning

Fix two tests that had illegal pointers to coarrays

Original-commit: flang-compiler/f18@36769996fa9559b84dda59f3f9eceee9d2269e74
Reviewed-on: https://github.com/flang-compiler/f18/pull/910