[flang] Fix BoxAnalyzer.h for LLVM_ENABLE_EXPENSIVE_CHECKS (llvm#96675)

Fixes assert seen trying to use default number of inlined elements for
`SmallVector<T>` but `sizeof(T)` is really big. Seen with
-DLLVM_ENABLE_EXPENSIVE_CHECKS.

Author: dpalermo

flang/include/flang/Lower/BoxAnalyzer.h

@@ -97,7 +97,7 @@ struct ScalarDynamicDerived : ScalarSym {
       : ScalarSym{sym}, lens{std::move(lens)} {}
 
 private:
-  llvm::SmallVector<Fortran::lower::SomeExpr> lens;
+  llvm::SmallVector<Fortran::lower::SomeExpr, 1> lens;
 };
 
 struct LBoundsAndShape {