[libcxx] [test] Remove nonportable locale assumption in basic.ios.members/narrow.pass.cpp

I'm not sure if libcxx is asserting UTF-8 here; but on Windows the full char value is always passed through in its entirety, since the default codepage is something like Windows-1252. The replacement character is only used for non-chars there; and that should be a more portable test everywhere.

(Still pending review at https://reviews.llvm.org/D47395 which has been open since may; will ask for forgiveness rather than permission :) )

llvm-svn: 339213