Remove the __log function for __log_level

Also redefine all of the standard logging macros to use more rust code instead
of custom LLVM translation code. This makes them a bit easier to understand, but
also more flexibile for future types of logging.

Additionally, this commit removes the LogType language item in preparation for
changing how logging is performed.
auto merge of #9005 : alexcrichton/rust/rusty-log, r=brson

Also redefine all of the standard logging macros to use more rust code instead
of custom LLVM translation code. This makes them a bit easier to understand, but
also more flexibile for future types of logging.

Additionally, this commit removes the LogType language item in preparation for
changing how logging is performed.