libstdc++: Replace operator>>(istream&, char*) [LWG 2499]

P0487R1 resolved LWG 2499 for C++20 by removing the operator>> overloads
that have high risk of buffer overflows. They were replaced by
equivalents that only accept a reference to an array, and so can
guarantee not to write past the end of the array.

In order to support both the old and new functionality, this patch
introduces a new overloaded __istream_extract function which takes a
maximum length. The new operator>> overloads use the array size as the
maximum length. The old overloads now use __builtin_object_size to
determine the available buffer size if available (which requires -O2) or
use numeric_limits<streamsize>::max()/sizeof(char_type) otherwise. This
is a change in behaviour, as the old overloads previously always used
numeric_limits<streamsize>::max(), without considering sizeof(char_type)
and without attempting to prevent overflows.

Because they now do little more than call __istream_extract, the old
operator>> overloads are very small inline functions. This means there
is no advantage to explicitly instantiating them in the library (in fact
that would prevent the __builtin_object_size checks from ever working).
As a result, the explicit instantiation declarations can be removed from
the header. The explicit instantiation definitions are still needed, for
backwards compatibility with existing code that expects to link to the
definitions in the library.

While working on this change I noticed that src/c++11/istream-inst.cc
has the following explicit instantiation definition:
  template istream& operator>>(istream&, char*);
This had no effect (and so should not have been present in that file),
because there was an explicit specialization declared in <istream> and
defined in src/++98/istream.cc. However, this change removes the
explicit specialization, and now the explicit instantiation definition
is necessary to ensure the symbol gets defined in the library.

libstdc++-v3/ChangeLog:

	* config/abi/pre/gnu.ver (GLIBCXX_3.4.29): Export new symbols.
	* include/bits/istream.tcc (__istream_extract): New function
	template implementing both of operator>>(istream&, char*) and
	operator>>(istream&, char(&)[N]). Add explicit instantiation
	declaration for it. Remove explicit instantiation declarations
	for old function templates.
	* include/std/istream (__istream_extract): Declare.
	(operator>>(basic_istream<C,T>&, C*)): Define inline and simply
	call __istream_extract.
	(operator>>(basic_istream<char,T>&, signed char*)): Likewise.
	(operator>>(basic_istream<char,T>&, unsigned char*)): Likewise.
	(operator>>(basic_istream<C,T>&, C(7)[N])): Define for LWG 2499.
	(operator>>(basic_istream<char,T>&, signed char(&)[N])):
	Likewise.
	(operator>>(basic_istream<char,T>&, unsigned char(&)[N])):
	Likewise.
	* include/std/streambuf (basic_streambuf): Declare char overload
	of __istream_extract as a friend.
	* src/c++11/istream-inst.cc: Add explicit instantiation
	definition for wchar_t overload of __istream_extract. Remove
	explicit instantiation definitions of old operator>> overloads
	for versioned-namespace build.
	* src/c++98/istream.cc (operator>>(istream&, char*)): Replace
	with __istream_extract(istream&, char*, streamsize).
	* testsuite/27_io/basic_istream/extractors_character/char/3.cc:
	Do not use variable-length array.
	* testsuite/27_io/basic_istream/extractors_character/char/4.cc:
	Do not run test for C++20.
	* testsuite/27_io/basic_istream/extractors_character/char/9555-ic.cc:
	Do not test writing to pointers for C++20.
	* testsuite/27_io/basic_istream/extractors_character/char/9826.cc:
	Use array instead of pointer.
	* testsuite/27_io/basic_istream/extractors_character/wchar_t/3.cc:
	Do not use variable-length array.
	* testsuite/27_io/basic_istream/extractors_character/wchar_t/4.cc:
	Do not run test for C++20.
	* testsuite/27_io/basic_istream/extractors_character/wchar_t/9555-ic.cc:
	Do not test writing to pointers for C++20.
	* testsuite/27_io/basic_istream/extractors_character/char/lwg2499.cc:
	New test.
	* testsuite/27_io/basic_istream/extractors_character/char/lwg2499_neg.cc:
	New test.
	* testsuite/27_io/basic_istream/extractors_character/char/overflow.cc:
	New test.
	* testsuite/27_io/basic_istream/extractors_character/wchar_t/lwg2499.cc:
	New test.
	* testsuite/27_io/basic_istream/extractors_character/wchar_t/lwg2499_neg.cc:
	New test.

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