Change hash_table to support a comparator type different from the

Change hash_table to support a comparator type different from the
value type stored in the hash table.  The 'find' functions now may
take a different type from the value type.  This requires introducing
a second typedef into the Descriptor conceptual type.  Change the
Descriptor concept to use typedefs value_type and compare_type instead
of T.  Change all users to match.

Add usage documentation to hash-table.h.

Tested on x86-64.

Index: gcc/ChangeLog

2012-10-25  Lawrence Crowl  <crowl@google.com>

	* hash-table.h: Add usage documentation.
	(template struct typed_free_remove): Clarify documentation.
	Rename template parameter.
	(struct typed_noop_remove): Likewise.
	(descriptor concept): Change typedef T to value_type.
	Add typedef compare_type.  Use more precise template parameter name,
	Descriptor instead of Descr.  Update users to match.
	(struct hash_table): Change 'find' parameters to use compare_type
	instead of the value type.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@192823 138bc75d-0d04-0410-961f-82ee72b054a4