[symbolizer] Build 'Request' object in single point. NFC

All control paths in executeCommand create Request object for use in
calls to 'print' function and do it identically. With this change the
Request object is created in a single point, which simplifies changing
implementation of Request class.

This is a prerequisite patch for implementation of symbol+offset lookup.

Differential Revision: https://reviews.llvm.org/D147115