Switch //common to use C++20 concepts. (#3665)

This removes the use of `enable_if` and tries to adopt concepts instead
of type traits when available.

The `ostream.h` change is a bit subtle as it adds a restriction not
previously in place -- that the stream is *contvertible* to
`std::ostream` as well as having it as a base class. This seems to match
the intent of the code.

The `hashing.h` code adds an implementation detail concept, and so I've
also clarified that the dispatch namespace is an internal one that isn't
part of the public API.

common/hashing_test.cpp

@@ -7,7 +7,7 @@
 #include <gmock/gmock.h>
 #include <gtest/gtest.h>
 
-#include <type_traits>
+#include <concepts>
 
 #include "llvm/ADT/Sequence.h"
 #include "llvm/ADT/StringExtras.h"
@@ -339,7 +339,8 @@ auto PrintFullWidthHex(llvm::raw_ostream& os, T value) {
                       static_cast<uint64_t>(value));
 }
 
-template <typename T, typename = std::enable_if_t<std::is_integral_v<T>>>
+template <typename T>
+  requires std::integral<T>
 auto operator<<(llvm::raw_ostream& os, HashedValue<T> hv)
     -> llvm::raw_ostream& {
   os << "hash " << hv.hash << " for value ";
@@ -347,9 +348,8 @@ auto operator<<(llvm::raw_ostream& os, HashedValue<T> hv)
   return os;
 }
 
-template <typename T, typename U,
-          typename = std::enable_if_t<std::is_integral_v<T>>,
-          typename = std::enable_if_t<std::is_integral_v<U>>>
+template <typename T, typename U>
+  requires std::integral<T> && std::integral<U>
 auto operator<<(llvm::raw_ostream& os, HashedValue<std::pair<T, U>> hv)
     -> llvm::raw_ostream& {
   os << "hash " << hv.hash << " for pair of ";