[flang][openmp] libc++ unordered_map build fix in flang openmp static analysis

Simply move the include of unordered_map from the .cpp file to the .h file

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

@@ -9,7 +9,6 @@
 #include "check-omp-structure.h"
 #include "flang/Parser/parse-tree.h"
 #include "flang/Semantics/tools.h"
-#include <unordered_map>
 
 namespace Fortran::semantics {
 

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

@@ -19,6 +19,8 @@
 #include "flang/Semantics/semantics.h"
 #include "llvm/Frontend/OpenMP/OMPConstants.h"
 
+#include <unordered_map>
+
 using OmpDirectiveSet = Fortran::common::EnumSet<llvm::omp::Directive,
     llvm::omp::Directive_enumSize>;