[libcxx] Move the private header __std_stream into the src subdir

[libcxx] Move the private header __std_stream into the src subdir

This header isn't used by any public header, so there shouldn't
be any need to install it or treat it as a heder.

Once it's part of the src subdirectory, I guess one could consider
giving it a more traditional name too.

Differential Revision: https://reviews.llvm.org/D147855