[lldb][DebugNamesDWARF] Also use mangled name when matching regex

When LLDB queries the debug names index with a regex, we should use the
`Mangled` class wrapper, which attempts to match regex first against the mangled
name and then against the demangled name. It is important to do so, otherwise
queries like `frame var --regex A::` would never work. This is what is done for
the Apple index as well.

This fixes test/API/lang/cpp/class_static/main.cpp when compiled with DWARF 5.

Differential Revision: https://reviews.llvm.org/D154617