[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-sections.c b/clang/test/AST/ast-dump-openmp-sections.c
index 330732b..ddfa0b6 100644
--- a/clang/test/AST/ast-dump-openmp-sections.c
+++ b/clang/test/AST/ast-dump-openmp-sections.c
@@ -18,6 +18,6 @@
 // CHECK-NEXT:     `-OMPSectionsDirective {{.*}} <line:9:1, col:21>
 // CHECK-NEXT:       `-CapturedStmt {{.*}} <line:10:3, col:7>
 // CHECK-NEXT:         `-CapturedDecl {{.*}} <<invalid sloc>> <invalid sloc>
-// CHECK-NEXT:           |-CompoundStmt {{.*}} <col:3, col:7> openmp_structured_block
+// CHECK-NEXT:           |-CompoundStmt {{.*}} <col:3, col:7>
 // CHECK-NEXT:           | `-NullStmt {{.*}} <col:5>
 // CHECK-NEXT:           `-ImplicitParamDecl {{.*}} <line:9:1> col:1 implicit __context 'struct (anonymous at {{.*}}ast-dump-openmp-sections.c:9:1) *const restrict'