PR other/90695 reduce testcase to remove library dependency

PR other/90695 reduce testcase to remove library dependency

This reproduces the original ICE fixed by r178857 (tested at r178852 and
r178860), without depending on a libstdc++ header that keeps changing.

The number of errors differs between C++14 and C++17 modes, so the fixed
test uses dg-excess-errors to match any number of them. The precise
errors aren't what's being tested for here anyway, the point of the test
is to verify the ICE in PR 50391 is fixed.

	PR other/90695
	* g++.dg/cpp0x/noexcept15.C: Remove dependency on library header.

From-SVN: r272121