Replace FoldingSet with DenseMap for instruction canonicalization. (#3979)

Switch from recursing into non-canonical instruction fields to
separately canonicalizing those fields. This means we now form canonical
`InstBlockId`s, `TypeBlockId`s, `IntId`s, `FloatId`s, and `BindNameId`s
at least in the cases when they're referenced by a constant instruction.

This reduces the overall runtime for @chandlerc's 10MLoC example by
27.5% on my machine.

common/hashing_test.cpp

@@ -283,6 +283,7 @@ struct HashableType {
 
   int ignored = 0;
 
+  // See common/hashing.h.
   friend auto CarbonHashValue(const HashableType& value, uint64_t seed)
       -> HashCode {
     Hasher hasher(seed);