[Sanitizer Common] Show command used to launch symbolizer process at high verbosity level.

Summary:
In preparation for writing a test for a bug fix we need to be able to
see the command used to launch the symbolizer process. This feature
will likely be useful for debugging how the Sanitizers use the
symbolizer in general.

This patch causes the command line used to launch the process to be
shown at verbosity level 3 and higher.

A small test case is included.

Reviewers: kubamracek, yln, vitalybuka, eugenis, kcc

Subscribers: #sanitizers, llvm-commits

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D77622