[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-cancellation-point.c b/clang/test/AST/ast-dump-openmp-cancellation-point.c
index 7090090..f47a4db 100644
--- a/clang/test/AST/ast-dump-openmp-cancellation-point.c
+++ b/clang/test/AST/ast-dump-openmp-cancellation-point.c
@@ -13,7 +13,7 @@
 // CHECK-NEXT:     `-OMPParallelDirective {{.*}} <line:4:1, col:21>
 // CHECK-NEXT:       `-CapturedStmt {{.*}} <line:5:3, line:7:3>
 // CHECK-NEXT:         `-CapturedDecl {{.*}} <<invalid sloc>> <invalid sloc> nothrow
-// CHECK-NEXT:           |-CompoundStmt {{.*}} <line:5:3, line:7:3> openmp_structured_block
+// CHECK-NEXT:           |-CompoundStmt {{.*}} <line:5:3, line:7:3>
 // CHECK-NEXT:           | `-OMPCancellationPointDirective {{.*}} <line:6:1, col:40> openmp_standalone_directive
 // CHECK-NEXT:           |-ImplicitParamDecl {{.*}} <line:4:1> col:1 implicit .global_tid. 'const int *const restrict'
 // CHECK-NEXT:           |-ImplicitParamDecl {{.*}} <col:1> col:1 implicit .bound_tid. 'const int *const restrict'