[VirtualFileSystem] Use map to stabilize iteration order

StringMap iteration order is not guaranteed to be deterministic
(https://llvm.org/docs/ProgrammersManual.html#llvm-adt-stringmap-h).

Tested by `TEST_F(InMemoryFileSystemTest, DirectoryIteration)`.

Author: MaskRay

llvm/lib/Support/VirtualFileSystem.cpp

@@ -43,6 +43,7 @@
 #include <cstdint>
 #include <iterator>
 #include <limits>
+#include <map>
 #include <memory>
 #include <optional>
 #include <string>
@@ -725,7 +726,7 @@ class InMemoryFileAdaptor : public File {
 
 class InMemoryDirectory : public InMemoryNode {
   Status Stat;
-  llvm::StringMap<std::unique_ptr<InMemoryNode>> Entries;
+  std::map<std::string, std::unique_ptr<InMemoryNode>> Entries;
 
 public:
   InMemoryDirectory(Status Stat)
@@ -741,7 +742,7 @@ class InMemoryDirectory : public InMemoryNode {
   UniqueID getUniqueID() const { return Stat.getUniqueID(); }
 
   InMemoryNode *getChild(StringRef Name) const {
-    auto I = Entries.find(Name);
+    auto I = Entries.find(Name.str());
     if (I != Entries.end())
       return I->second.get();
     return nullptr;