[debuginfo-tests][dexter] Add two new debug experience tests

deferred_globals.cpp: Verify that debug information for a local variable does
not hide a global definition that has the same name

namespace.cpp: Ensure that the debug information for a global variable
includes namespace information.

Differential Revision: https://reviews.llvm.org/D89462

Author:    Nabeel Omer <nabeel.omer@sony.com>