[flang][OpenMP] Fix LINEAR clause validation and test expectations (#175707)
Fixes #175688
After #175383 was merged, the test
`Semantics/OpenMP/linear-clause01.f90` was failing because it had an
early return that prevented multiple errors from being reported.
This PR fixes two issues:
1. **Removes the early return** after detecting a modifier error on
DO/SIMD directives. Previously, when a modifier error was found, the
function would return immediately without checking other restrictions
like the scalar requirement. Now all applicable errors are reported.
2. **Updates test expectations** to expect both the modifier error AND
the scalar error for Case 1, where `arg(:)` is an array used with `uval`
modifier on a DO directive.
Thanks to @NimishMishra for catching this during review - arrays should
always be rejected in LINEAR clauses (except for `declare simd` with
`ref` modifier), regardless of whether there are other errors.