[libc++][format] Enable format_error on older compilers.

It seems like modifying the header doesn't cause libc++ to be rebuild.
So the breakage of the previous commit didn't happen on my system.

This should fix the build of https://buildkite.com/mlir/mlir-core

GitOrigin-RevId: f8772da8cc9a0be65c9ba028c2b5a895c1ed4f91
1 file changed