[flang] Allow exterior branch to outermost WHERE construct statement

[klausler]

Summary

The labels of WHERE constructs were being created within the scope of
the construct, not the scope of its parent, leading to incorrect error
messages for branches to that label.

[PeteSteinfeld]

Looks great!

flang/lib/Semantics/resolve-labels.cpp

template <typename A> bool Pre(const parser::Statement<A> &statement) {
     currentPosition_ = statement.source;
     const auto &label = statement.label;
     if (!label) {
       return true;
     }
     using LabeledConstructStmts = std::tuple<parser::AssociateStmt,
         parser::BlockStmt, parser::ChangeTeamStmt, parser::CriticalStmt,
         parser::IfThenStmt, parser::NonLabelDoStmt, parser::SelectCaseStmt,
-        parser::SelectRankStmt, parser::SelectTypeStmt>;
+        parser::SelectRankStmt, parser::SelectTypeStmt,
+        parser::WhereConstructStmt>;
     using LabeledConstructEndStmts = std::tuple<parser::EndAssociateStmt,
         parser::EndBlockStmt, parser::EndChangeTeamStmt,
         parser::EndCriticalStmt, parser::EndDoStmt, parser::EndForallStmt,
         parser::EndIfStmt, parser::EndSelectStmt, parser::EndWhereStmt>;
     using LabeledProgramUnitEndStmts =
         std::tuple<parser::EndFunctionStmt, parser::EndMpSubprogramStmt,
             parser::EndProgramStmt, parser::EndSubroutineStmt>;
     auto targetFlags{ConstructBranchTargetFlags(statement)};