Define __cpp_lib_erase_if feature test macro

Define __cpp_lib_erase_if feature test macro

The C++2a draft specifies the value 201811L for this, but as an
extension we return the number of elements erased. This is expected to
be standardised, so the macro has the value 201900L until a proper value
is specified in the draft.

	* include/bits/erase_if.h: Define __cpp_lib_erase_if.
	* include/std/deque: Likewise.
	* include/std/forward_list: Likewise.
	* include/std/list: Likewise.
	* include/std/string: Likewise.
	* include/std/vector: Likewise.
	* include/std/version: Likewise.
	* testsuite/21_strings/basic_string/erasure.cc: Test macro.
	* testsuite/23_containers/deque/erasure.cc: Likewise.
	* testsuite/23_containers/forward_list/erasure.cc: Likewise.
	* testsuite/23_containers/list/erasure.cc: Likewise.
	* testsuite/23_containers/map/erasure.cc: Likewise.
	* testsuite/23_containers/set/erasure.cc: Likewise.
	* testsuite/23_containers/unordered_map/erasure.cc: Likewise.
	* testsuite/23_containers/unordered_set/erasure.cc: Likewise.
	* testsuite/23_containers/vector/erasure.cc: Likewise.

From-SVN: r267810