Starting using murmur2 when combining multiple size_t's into a single hash, and also for basic_string. Also made hash<thread::id> ever so slighly more portable. I had to tweak one test which is questionable (definitely not portable) anyway.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@145795 91177308-0d34-0410-b5e6-96231b3b80d8

test/utilities/function.objects/unord.hash/integral.pass.cpp

@@ -33,7 +33,8 @@ test()
     for (int i = 0; i <= 5; ++i)
     {
         T t(i);
-        assert(h(t) == t);
+        if (sizeof(T) <= sizeof(std::size_t))
+            assert(h(t) == t);
     }
 }