Add static_asserts to basic_ios and basic_stream_buf to ensure that that the...

Add static_asserts to basic_ios and basic_stream_buf to ensure that that the traits match the character type. This is a requirement on the user - now we get consistent failures at compile time instead of incomprehensible error messages or runtime failures. This is also LWG#2994 - not yet adopted.

llvm-svn: 323945