[CIR][CodeGen] Use MapVector instead of StringMap for Replacements (#2116)

Fix #1938

Minimized benchmark:
```cpp
#include <vector>

class my_class {
public:
    std::vector<void*> my_vec;
};

my_class inst;
```

When using `llvm::StringMap` transitive replacements may be out of
order. So use `llvm::MapVector` which is also used in clang's CodeGen.

https://github.com/llvm/clangir/blob/0c130cda66d792752fa52f57aabdf561a49482fd/clang/lib/CodeGen/CodeGenModule.h#L439

I couldn't reproduce the MRE with `#include "std-cxx.h"`, any other way
to add the test?

Author: xaerru

clang/lib/CIR/CodeGen/CIRGenModule.cpp

@@ -4268,7 +4268,7 @@ void CIRGenModule::replacePointerTypeArgs(cir::FuncOp oldF, cir::FuncOp newF) {
 
 void CIRGenModule::applyReplacements() {
   for (auto &i : Replacements) {
-    StringRef mangledName = i.first();
+    StringRef mangledName = i.first;
     mlir::Operation *replacement = i.second;
     auto *entry = getGlobalValue(mangledName);
     if (!entry)

clang/lib/CIR/CodeGen/CIRGenModule.h

@@ -1022,8 +1022,7 @@ class CIRGenModule : public CIRGenTypeCache {
   llvm::StringMap<clang::GlobalDecl, llvm::BumpPtrAllocator> Manglings;
 
   // FIXME: should we use llvm::TrackingVH<mlir::Operation> here?
-  typedef llvm::StringMap<mlir::Operation *> ReplacementsTy;
-  ReplacementsTy Replacements;
+  llvm::MapVector<StringRef, mlir::Operation *> Replacements;
   /// Call replaceAllUsesWith on all pairs in Replacements.
   void applyReplacements();