[lldb] make ConstStringTable use DenseMap rather than std::map

The ordering is not needed, and DenseMap is faster. I can measure
time spent in the SaveToCache() calls reduced to ~40% during LLDB
startup (and the total startup cost reduced to ~70%).

Differential Revision: https://reviews.llvm.org/D122980

lldb/include/lldb/Core/DataFileCache.h

@@ -13,6 +13,7 @@
 #include "lldb/Utility/Status.h"
 #include "lldb/Utility/UUID.h"
 #include "lldb/lldb-forward.h"
+#include "llvm/ADT/DenseMap.h"
 #include "llvm/Support/Caching.h"
 #include <mutex>
 
@@ -190,7 +191,7 @@ public:
 
 private:
   std::vector<ConstString> m_strings;
-  std::map<ConstString, uint32_t> m_string_to_offset;
+  llvm::DenseMap<ConstString, uint32_t> m_string_to_offset;
   /// Skip one byte to start the string table off with an empty string.
   uint32_t m_next_offset = 1;
 };