Add LLDB_LOG_ERROR macro

Summary:
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

llvm-svn: 323753