[clang][NFC] Move static assert on StmtClass bounds from header to source

A follow-up change to PR #120341 & #120643.  Address @nikic's concern:
#120341 (comment)

Author: ziqingluo-90

clang/include/clang/AST/Stmt.h

@@ -109,16 +109,7 @@ class alignas(void *) Stmt {
 
   //===--- Statement bitfields classes ---===//
 
-  enum { NumStmtBits = 9 };
-
-#define STMT(CLASS, PARENT)
-#define STMT_RANGE(BASE, FIRST, LAST)
-#define LAST_STMT_RANGE(BASE, FIRST, LAST)                                     \
-  static_assert(llvm::isUInt<NumStmtBits>(StmtClass::LAST##Class),             \
-                "The number of 'StmtClass'es is strictly bound "               \
-                "by a bitfield of width NumStmtBits");
-#define ABSTRACT_STMT(STMT)
-#include "clang/AST/StmtNodes.inc"
+  #define NumStmtBits 9
 
   class StmtBitfields {
     friend class ASTStmtReader;

clang/lib/AST/Stmt.cpp

@@ -47,6 +47,15 @@
 
 using namespace clang;
 
+#define STMT(CLASS, PARENT)
+#define STMT_RANGE(BASE, FIRST, LAST)
+#define LAST_STMT_RANGE(BASE, FIRST, LAST)                                     \
+  static_assert(llvm::isUInt<NumStmtBits>(Stmt::StmtClass::LAST##Class),             \
+                "The number of 'StmtClass'es is strictly bound "               \
+                "by a bitfield of width NumStmtBits");
+#define ABSTRACT_STMT(STMT)
+#include "clang/AST/StmtNodes.inc"
+
 static struct StmtClassNameTable {
   const char *Name;
   unsigned Counter;