Add methods to enable using formatv syntax in LLDB.

This adds formatv-backed formatting functions in various
places in LLDB such as StreamString, logging, constructing
error messages, etc.  A couple of callsites are changed
from Printf style syntax to formatv style syntax to
illustrate its usage.  Additionally, a FileSpec formatter
is introduced so that FileSpecs can be formatted natively.

Differential Revision: https://reviews.llvm.org/D27632

llvm-svn: 289922