c++: add test for DR 2855

c++: add test for DR 2855

Let

  int8_t x = 127;

This DR says that while

  x++;

invokes UB,

  ++x;

does not.  The resolution was to make the first one valid.  The
following test verifies that we don't report any errors in a constexpr
context.

	DR 2855

gcc/testsuite/ChangeLog:

	* g++.dg/DRs/dr2855.C: New test.