[MemProf] Switch to DenseMap for performance (NFC) (llvm#81035)

Some profiling showed that the accesses to this map during bitcode
reading was incurring over 10% of the time in a large thin link. There
is no need for it to be std::map, and I measured around 8.5% time
reduction in the same thin link from switching to DenseMap.

llvm/include/llvm/IR/ModuleSummaryIndex.h

@@ -1377,7 +1377,7 @@ class ModuleSummaryIndex {
 
   // Temporary map while building StackIds list. Clear when index is completely
   // built via releaseTemporaryMemory.
-  std::map<uint64_t, unsigned> StackIdToIndex;
+  DenseMap<uint64_t, unsigned> StackIdToIndex;
 
   // YAML I/O support.
   friend yaml::MappingTraits<ModuleSummaryIndex>;