[libc++] Update clang-format style.

After evaluating the new style I noticed inner namespaces are now
indented. I am not fond of that style and I've seen some other review
comment in this regard so I propose we remove this option and use the
LLVM default not to indent it.

Reviewed By: ldionne, philnik, var-const, #libc

Differential Revision: https://reviews.llvm.org/D129441