[flang] Add missing std includes (#211830)

Addressing post-commit review comments for
https://github.com/llvm/llvm-project/pull/211326.

flang/lib/Semantics/check-acc-structure.h

@@ -21,6 +21,11 @@
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/Frontend/OpenACC/ACC.h.inc"
 
+#include <cstddef>
+#include <functional>
+#include <optional>
+#include <string>
+
 using AccDirectiveSet = Fortran::common::EnumSet<llvm::acc::Directive,
     llvm::acc::Directive_enumSize>;
 

flang/lib/Semantics/check-directive-structure.h

@@ -12,7 +12,6 @@
 #ifndef FORTRAN_SEMANTICS_CHECK_DIRECTIVE_STRUCTURE_H_
 #define FORTRAN_SEMANTICS_CHECK_DIRECTIVE_STRUCTURE_H_
 
-#include "flang/Common/enum-set.h"
 #include "flang/Semantics/semantics.h"
 #include "flang/Semantics/tools.h"
 #include "llvm/ADT/iterator_range.h"

flang/lib/Semantics/check-omp-structure.h

@@ -23,6 +23,18 @@
 #include "llvm/ADT/iterator_range.h"
 #include "llvm/Frontend/OpenMP/OMP.h"
 
+#include <cstddef>
+#include <functional>
+#include <list>
+#include <map>
+#include <optional>
+#include <set>
+#include <string>
+#include <string_view>
+#include <utility>
+#include <variant>
+#include <vector>
+
 #define GEN_FLANG_DIRECTIVE_CLAUSE_SETS
 #include "llvm/Frontend/OpenMP/OMP.inc"