PR libstdc++/89090 avoid C++17 features in C++11/C++14 code

PR libstdc++/89090 avoid C++17 features in C++11/C++14 code

Although GCC and Clang both allow these features pre-C++17 in system
headers, Clang does issue warnings with -Wsystem-headers. It can also
complicate bisection and/or testcase reduction if # line markers are
stripped, because the code won't be known to come from system headers.

	PR libstdc++/89090
	* include/bits/stl_uninitialized.h (__relocate_a_1): Make unused
	parameter unnamed. Add message to static assertion.
	* include/bits/vector.tcc (vector::reserve, vector::_M_realloc_insert)
	(vector::_M_default_append): Use _GLIBCXX17_CONSTEXPR for if constexpr
	in C++11 code.

From-SVN: r268536