Several of the pb_ds headers are intended to be included multiple times,
within the definition of various class templates. The including files
define macros like PB_DS_CLASS_C_DEC and PB_DS_GEN_POS before including
these headers.

In some cases the types defined in the headers are actually nested types
within other classes, and so should not have been documented as though
they are declared in the global namespace, as in:
https://gcc.gnu.org/onlinedocs/gcc-8.3.0/libstdc++/api/a12028.html

In other cases the headers provide inline member function definitions,
but when processed by Doxygen the class name "PB_DS_CLASS_C_DEC" is not
recognised.

This patch makes Doxygen ignore definitions that only make sense when
included in the right context with the right macros defined.

	* include/ext/pb_ds/detail/bin_search_tree_/*_imps.hpp: Do not define
	anything unless PB_DS_CLASS_C_DEC is defined.
	* include/ext/pb_ds/detail/binary_heap_/*_imps.hpp: Likewise.
	* include/ext/pb_ds/detail/binomial_heap_/*_imps.hpp: Likewise.
	* include/ext/pb_ds/detail/binomial_heap_base_/*_imps.hpp: Likewise.
	* include/ext/pb_ds/detail/cc_hash_table_map_/*_imps.hpp: Likewise.
	* include/ext/pb_ds/detail/gp_hash_table_map_/*_imps.hpp: Likewise.
	* include/ext/pb_ds/detail/hash_fn/*_imp.hpp: Likewise.
	* include/ext/pb_ds/detail/left_child_next_sibling_heap_/*_imps.hpp:
	Likewise.
	* include/ext/pb_ds/detail/list_update_map_/*_imps.hpp: Likewise.
	* include/ext/pb_ds/detail/ov_tree_map_/*_imps.hpp: Likewise.
	* include/ext/pb_ds/detail/pairing_heap_/*_imps.hpp: Likewise.
	* include/ext/pb_ds/detail/pat_trie_/*_imps.hpp: Likewise.
	* include/ext/pb_ds/detail/rb_tree_map_/*_imps.hpp: Likewise.
	* include/ext/pb_ds/detail/rc_binomial_heap_/*_imps.hpp: Likewise.
	* include/ext/pb_ds/detail/resize_policy*_imp.hpp: Likewise.
	* include/ext/pb_ds/detail/splay_tree_/*_imps.hpp: Likewise.
	* include/ext/pb_ds/detail/thin_heap_/*_imps.hpp: Likewise.
	* include/ext/pb_ds/detail/trie_policy*_imp.hpp: Likewise.
	* include/ext/pb_ds/detail/unordered_iterator/const_iterator.hpp:
	Likewise.
	* include/ext/pb_ds/detail/unordered_iterator/iterator.hpp: Likewise.
	* include/ext/pb_ds/detail/unordered_iterator/point_const_iterator.hpp:
	Likewise.
	* include/ext/pb_ds/detail/unordered_iterator/point_iterator.hpp:
	Likewise.

From-SVN: r270803