[coro] Fix crash due to DILabel in LineTableOnly mode

Since the recent commit de3c841, the `CoroSplit` pass adds `DILabel`s
to help find the location of a suspension point from its suspension
point id. Those labels are added in both `DebugEmissionKind::FullDebug`
and `DebugEmissionKind::LineTableOnly` mode. The idea was that this
information is necessary to reconstruct async stack traces and should
hence also be available for LineTableOnly.

Unfortunately, it turns out that the DWARF backend does not expect to
find any DILabel debug metadata if the emission kind is set to
LineTableOnly. The Dwarf backend simply runs into an assertion in that
case.

This commit fixes the issue by only adding `DILabel` for FullDebug.

Author: vogelsgesang

llvm/lib/Transforms/Coroutines/CoroSplit.cpp

@@ -1484,12 +1484,9 @@ struct SwitchCoroutineSplitter {
     // If there is no DISubprogram for F, it implies the function is compiled
     // without debug info. So we also don't generate debug info for the
     // suspension points.
-    bool AddDebugLabels =
-        (DIS && DIS->getUnit() &&
-         (DIS->getUnit()->getEmissionKind() ==
-              DICompileUnit::DebugEmissionKind::FullDebug ||
-          DIS->getUnit()->getEmissionKind() ==
-              DICompileUnit::DebugEmissionKind::LineTablesOnly));
+    bool AddDebugLabels = DIS && DIS->getUnit() &&
+                          (DIS->getUnit()->getEmissionKind() ==
+                           DICompileUnit::DebugEmissionKind::FullDebug);
 
     // resume.entry:
     //  %index.addr = getelementptr inbounds %f.Frame, %f.Frame* %FramePtr, i32