libstdc++: Implement P0966 std::string::reserve should not shrink

libstdc++: Implement P0966 std::string::reserve should not shrink

Remove ability for reserve(n) to reduce a string's capacity. Add a new
reserve() overload that makes a shrink-to-fit request, and make
shrink_to_fit() use that.

libstdc++-v3/ChangeLog:

2020-07-30  Andrew Luo  <andrewluotechnologies@outlook.com>
	    Jonathan Wakely  <jwakely@redhat.com>

	* config/abi/pre/gnu.ver (GLIBCXX_3.4): Use less greedy
	patterns for basic_string members.
	(GLIBCXX_3.4.29): Export new basic_string::reserve symbols.
	* doc/xml/manual/status_cxx2020.xml: Update P0966 status.
	* include/bits/basic_string.h (shrink_to_fit()): Call reserve().
	(reserve(size_type)): Remove default argument.
	(reserve()): Declare new overload.
	[!_GLIBCXX_USE_CXX11_ABI] (shrink_to_fit, reserve): Likewise.
	* include/bits/basic_string.tcc (reserve(size_type)): Remove
	support for shrinking capacity.
	(reserve()): Perform shrink-to-fit operation.
	[!_GLIBCXX_USE_CXX11_ABI] (reserve): Likewise.
	* testsuite/21_strings/basic_string/capacity/1.cc: Adjust to
	reflect new behavior.
	* testsuite/21_strings/basic_string/capacity/char/1.cc:
	Likewise.
	* testsuite/21_strings/basic_string/capacity/char/18654.cc:
	Likewise.
	* testsuite/21_strings/basic_string/capacity/char/2.cc:
	Likewise.
	* testsuite/21_strings/basic_string/capacity/wchar_t/1.cc:
	Likewise.
	* testsuite/21_strings/basic_string/capacity/wchar_t/18654.cc:
	Likewise.
	* testsuite/21_strings/basic_string/capacity/wchar_t/2.cc:
	Likewise.