libstdc++: Update guards on __heterogeneous_hash_key and __heterogeneous_tree_key

libstdc++: Update guards on __heterogeneous_hash_key and __heterogeneous_tree_key

The r16-7359-gae04c1afd1526a changed __heterogeneous_key to be
defined if __glibcxx_associative_heterogeneous_erasure, but missed
guards on derived __heterogeneous_hash_key and __heterogeneous_tree_key
concept.

libstdc++-v3/ChangeLog:

	* include/bits/hashtable.h (std::__heterogeneous_hash_key)
	[__glibcxx_associative_heterogeneous_erasure]: Changed guard.
	* include/bits/stl_tree.h (std::__heterogeneous_tree_key)
	[__glibcxx_associative_heterogeneous_erasure]: Likewise.
	* include/bits/stl_function.h: Add comment with C++ version
	for __glibcxx_associative_heterogeneous_erasure guard.