Fix TestDataFormatterUnordered for older libc++ versions

clang recently started diagnosing "exception specification in
declaration does not match previous declaration" errors. Unfortunately
old libc++ versions had a bug, where they violated this rule, which
means that tests using this library version now fail due to build
errors.

Since it was easy to work around the bug by compiling this test with
-fno-exceptions, I do that here. If supporting old libc++ versions
becomes a burden, we'll have to revisit this.

llvm-svn: 337173