[flang][OpenMP] Split check-omp-structure.cpp into smaller files, NFC (#146359)

Create these new files in flang/lib/Semantics:
  openmp-utils.cpp/.h         - Common utilities
  check-omp-atomic.cpp        - Atomic-related checks
  check-omp-loop.cpp          - Loop constructs/clauses
  check-omp-metadirective.cpp - Metadirective-related checks

Update lists of included headers, std in particular.

---------

Co-authored-by: Jack Styles <[email protected]>

Author: kparzysz

flang/lib/Semantics/CMakeLists.txt

@@ -20,6 +20,9 @@ add_flang_library(FortranSemantics
   check-io.cpp
   check-namelist.cpp
   check-nullify.cpp
+  check-omp-atomic.cpp
+  check-omp-loop.cpp
+  check-omp-metadirective.cpp
   check-omp-structure.cpp
   check-purity.cpp
   check-return.cpp
@@ -34,12 +37,13 @@ add_flang_library(FortranSemantics
   mod-file.cpp
   openmp-dsa.cpp
   openmp-modifiers.cpp
+  openmp-utils.cpp
   pointer-assignment.cpp
   program-tree.cpp
-  resolve-labels.cpp
   resolve-directives.cpp
-  resolve-names-utils.cpp
+  resolve-labels.cpp
   resolve-names.cpp
+  resolve-names-utils.cpp
   rewrite-parse-tree.cpp
   runtime-type-info.cpp
   scope.cpp