P1236R1 - Signed integers are two's complement
gcc/cp/
	* constexpr.c (cxx_eval_check_shift_p): Disable the signed LSHIFT_EXPR
	checks for c++2a.
gcc/c-family/
	* c-warn.c (maybe_warn_shift_overflow): Don't warn for c++2a.
	* c-ubsan.c (ubsan_instrument_shift): Make signed shifts
	with in-range second operand well defined for -std=c++2a.
gcc/
	* doc/invoke.texi (Wshift-overflow): Adjust documentation for
	c++2a.
gcc/testsuite/
	* g++.dg/cpp2a/constexpr-shift1.C: New test.
	* g++.dg/warn/permissive-1.C (enum A, enum D): Don't expect
	diagnostics here for c++2a.
	* g++.dg/cpp0x/constexpr-shift1.C (fn3, i3, fn4, i4): Don't expect
	diagnostics here for c++2a.
	* g++.dg/cpp0x/constexpr-60049.C (f3, x3, y3): Likewise.
	* g++.dg/ubsan/cxx11-shift-1.C (main): Add some further tests.
	* g++.dg/ubsan/cxx11-shift-2.C (main): Likewise.
	* g++.dg/ubsan/cxx2a-shift-1.C: New test.
	* g++.dg/ubsan/cxx2a-shift-2.C: New test.

From-SVN: r266153