[flang][OpenMP] Add explicit N to SmallVector to avoid size error

This fixes https://lab.llvm.org/buildbot/#/builders/268/builds/13925,
which somehow doesn't show in any of my local builds.

Author: kparzysz

llvm/include/llvm/Frontend/OpenMP/ConstructCompositionT.h

@@ -297,7 +297,7 @@ template <typename C> void ConstructCompositionT<C>::mergeDSA() {
   };
 
   // Use ordered containers to avoid non-deterministic output.
-  llvm::SmallVector<std::pair<ObjectTy, int>> objectDsa;
+  llvm::SmallVector<std::pair<ObjectTy, int>, 8> objectDsa;
 
   auto getDsa = [&](const ObjectTy &object) -> std::pair<ObjectTy, int> & {
     auto found = llvm::find_if(objectDsa, [&](std::pair<ObjectTy, int> &p) {