[clang] Prune 'IsOMPStructuredBlock' Stmt bit

As discussed in https://reviews.llvm.org/D59214#1916596
and in some other reviews dealing with FPenv,
bits in Stmt are scarce, and i got so burnout with D59214
and https://bugs.llvm.org/show_bug.cgi?id=40563 specifically
that i never actually followed up with the usages for this bit.

So let's unhoard it, at least for now?
diff --git a/clang/test/AST/ast-dump-openmp-atomic.c b/clang/test/AST/ast-dump-openmp-atomic.c
index de9526e..53d3306 100644
--- a/clang/test/AST/ast-dump-openmp-atomic.c
+++ b/clang/test/AST/ast-dump-openmp-atomic.c
@@ -12,7 +12,7 @@
 // CHECK-NEXT:     `-OMPAtomicDirective {{.*}} <line:4:1, col:19>
 // CHECK-NEXT:       `-CapturedStmt {{.*}} <line:5:3, col:5>
 // CHECK-NEXT:         |-CapturedDecl {{.*}} <<invalid sloc>> <invalid sloc>
-// CHECK-NEXT:         | |-UnaryOperator {{.*}} <col:3, col:5> openmp_structured_block 'int' prefix '++'
+// CHECK-NEXT:         | |-UnaryOperator {{.*}} <col:3, col:5> 'int' prefix '++'
 // CHECK-NEXT:         | | `-DeclRefExpr {{.*}} <col:5> 'int' lvalue ParmVar {{.*}} 'i' 'int'
 // CHECK-NEXT:         | `-ImplicitParamDecl {{.*}} <line:4:1> col:1 implicit __context 'struct (anonymous at {{.*}}ast-dump-openmp-atomic.c:4:1) *const restrict'
 // CHECK-NEXT:         `-DeclRefExpr {{.*}} <line:5:5> 'int' lvalue ParmVar {{.*}} 'i' 'int'