Reland "[lldb][DWARFASTParserClang] Attach linkage name to ctors/dtors if missing"

This relands the commit previously reverted in
`d2cc2c56` due to failures on Linux
when debugging split-debug-info enabled executables.

The problem was we called `SymbolFileDWARF::FindFunctions` directly
instead of `Module::FindFunctions` which resulted in a nullptr
dereference because the backing `SymbolFileDWARFDwo` didn't have
an index attached to it. The relanded version calls `Module::FindFunctions`
instead.

Differential Revision: https://reviews.llvm.org/D143652