[lldb/IRExecutionUnit] Stop searching based on demangled names

Summary:
This was causing problems on linux, where we'd end up calling the
deleting destructor instead of a regular one (because they have the same
demangled name), making a lot of mischief in the process.

The only place where this was necessary (according to the test suite, at
least) was to call a base structor instead of a complete one, but this
is now handled in a more targeted fashion.

TestCallOverriddenMethod is now re-enabled as it now passes reliably.

Reviewers: teemperor, JDevlieghere

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D70722