[clang][DebugInfo] Emit unified (Itanium) mangled name to structor declarations (#154142)

Depends on https://github.com/llvm/llvm-project/pull/154137

This patch is motivated by
https://github.com/llvm/llvm-project/pull/149827, where we plan on using
mangled names on structor declarations to find the exact structor
definition that LLDB's expression evaluator should call.

Given a `DW_TAG_subprogram` for a function declaration, the most
convenient way for a debugger to find the corresponding definition is to
use the `DW_AT_linkage_name` (i.e., the mangled name). However, we
currently can't do that for constructors/destructors because Clang
doesn't attach linkage names to them. This is because, depending on ABI,
there can be multiple definitions for a single constructor/destructor
declaration. The way GCC works around this is by producing a `C4`/`D4`
"unified" mangling for structor declarations (see
[godbolt](https://godbolt.org/z/Wds6cja9K)). GDB uses this to locate the
relevant definitions.

This patch aligns Clang with GCC's DWARF output and allows us to
implement the same lookup scheme in LLDB.

1 files changed, 2 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CGClass.cpp b/clang/lib/CodeGen/CGClass.cpp
index bae55aa..6742c19 100644
--- a/clang/lib/CodeGen/CGClass.cpp
+++ b/clang/lib/CodeGen/CGClass.cpp
@@ -1481,6 +1481,8 @@ void CodeGenFunction::EmitDestructorBody(FunctionArgList &Args) {
   // we'd introduce *two* handler blocks.  In the Microsoft ABI, we
   // always delegate because we might not have a definition in this TU.
   switch (DtorType) {
+  case Dtor_Unified:
+    llvm_unreachable("not expecting a unified dtor");
   case Dtor_Comdat: llvm_unreachable("not expecting a COMDAT");
   case Dtor_Deleting: llvm_unreachable("already handled deleting case");