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

**Summary**

This patch addresses the case where we have a `DW_AT_external`
subprogram for a constructor (and/or destructor) that doesn't carry
a `DW_AT_linkage_name` attribute. The corresponding DIE(s) that
represent the definition will have a linkage name, but if the name
contains constructs that LLDBs fallback mechanism for guessing mangled
names to resolve external symbols doesn't support (e.g., abi-tags)
then we end up failing to resolve the function call.

We address this by trying to find the linkage name before we create
the constructor/destructor decl, which will get attached using
an `AsmLabelAttr` to make symbol resolution easier.

**Testing**

* Added API test

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