Improved expression logging. Now all calls to

FindExternalVisibleDecls and FindExternalLexicalDecls
are marked and given unique IDs, so that all logging
done as part of their execution can be traced back to
the proper call.

Also there was some logging that really wasn't helpful
in most cases so I disabled it unless verbose logging
(log enable -v lldb expr) is enabled.

llvm-svn: 141987