Add LLDB_LOG_ERROR macro
The difference between this and regular LLDB_LOG is that this one clears
the error object unconditionally. This was inspired by the
ObjectFileELF bug (r322664), where the error object was being cleared
only if logging was enabled.
Reviewers: davide, zturner, jingham, clayborg
Subscribers: lldb-commits, emaste
Differential Revision: https://reviews.llvm.org/D42182