[flang] Fix dangling pointer in LabelEnforce

`DirectiveStructureChecker` was passing in a pointer to a temporary
string for the `construct` argument to the constructor for `LabelEnforce`.
The `LabelEnforce` object had a lifetime longer than the temporary,
resulting in accessing a dangling pointer when emitting an error message
for `omp-parallell01.f90`.

The fix is to make the lifetime of the temporary as long as the lifetime
of the `LabelEnforce` object.

Differential Revision: https://reviews.llvm.org/D94618