Remove LIBLLDB_LOG_VERBOSE category

Summary:
Per discussion in D28616, having two ways two request logging (log
enable lldb XXX verbose && log enable -v lldb XXX) is confusing. This
removes the first option and standardizes all code to use the second
one.

I've added a LLDB_LOGV macro as a shorthand for if(log &&
log->GetVerbose()) and switched most of the affected log statements to
use that (I've only left a couple of cases that were doing complex
computations in an if(log) block).

Reviewers: jingham, zturner

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D29510

llvm-svn: 294113