[libcxx] Set LC_ALL rather than LC_COLLATE to override collation.

r251131 replaced LANG with LC_COLLATE. But LC_ALL has precedence
over both, so the test still fails when LC_ALL=C.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@257018 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed