[lldb] Explicitly open file to write with utf-8 encoding in crashlog.py

The python "open" function will use the default encoding for the
locale (the result of "locale.getpreferredencoding()"). Explicitly set
the locale to utf-8 when opening the crashlog for writing, as there may
be non-ascii symbols in there (for example, Swift uses "τ" to indicate
generic parameters).

rdar://101402755

Differential Revision: https://reviews.llvm.org/D136798