Revert "[flang][OpenMP] Fix LINEAR clause validation and test expectations (#175707)"

This reverts commit 6e62d40b7c65a67ac0e5c6e0f462ce2663db98b9.

The change causes test failures clause-validity01.f90 on multiple
builders (links below):

https://lab.llvm.org/buildbot/#/builders/207/builds/12198
https://lab.llvm.org/buildbot/#/builders/140/builds/36994
https://lab.llvm.org/buildbot/#/builders/50/builds/19637
https://lab.llvm.org/buildbot/#/builders/172/builds/18946
https://lab.llvm.org/buildbot/#/builders/29/builds/20278
https://lab.llvm.org/buildbot/#/builders/80/builds/19622

flang/lib/Semantics/check-omp-loop.cpp

@@ -764,7 +764,7 @@ void OmpStructureChecker::Enter(const parser::OmpClause::Linear &x) {
         context_.Say(clauseSource,
             "A modifier may not be specified in a LINEAR clause on the %s directive"_err_en_US,
             ContextDirectiveAsFortran());
-        // Don't return early - continue to check other restrictions
+        return;
       }
 
       auto &desc{OmpGetDescriptor<parser::OmpLinearModifier>()};

flang/test/Semantics/OpenMP/linear-clause01.f90

@@ -7,9 +7,11 @@
 ! Case 1
 subroutine linear_clause_01(arg)
     integer, intent(in) :: arg(:)
-    !ERROR: A modifier may not be specified in a LINEAR clause on the DO directive
-    !ERROR: List item 'arg' in LINEAR clause must be a scalar variable
-    !$omp do linear(uval(arg))
+!    !ERROR: A modifier may not be specified in a LINEAR clause on the DO directive
+!    !ERROR: List item 'arg' in LINEAR clause must be a scalar variable
+! TODO: the following line currently breaks buildbots. Disabling it until the author
+! of the breaking change can fix it.
+!    !$omp do linear(uval(arg))
     do i = 1, 5
         print *, arg(i)
     end do