[MLIR][LLVM] Exporter skip over inlined frame without debug scope (#90915)

Followup to #90759.

Instead of just returning null when the caller scope is not
translatable, "jump over" the current caller scope and use the outer
scope as the caller if that is available. This means that in an inlined
call stack if there are frames without debug scope, those frames are
skipped to preserve what is available. In the original example where
```
func A {
  foo loc(fused<#A>["a":1:1])
}
func B {
  call @A loc("b":1:1)
}
func C {
  call @b loc(fused<#C>["c":1:1])
}
```
is inlined into
```
func C {
  foo loc(callsite(
            callsite(fused<#A>["a":1:1] at loc("b":1:1))
           at
            fused<#C>["c":1:1]))
}
```
The translated result would be `!1`:
```
!0 = !DILocation(line: 1, column: 1, scope: !C)
!1 = !DILocation(line: 1, column: 1, scope: !A, inlinedAt: !0)
```

This has a neat benefit in maintaining callsite associativity: No matter
if we have `callsite(callsite(A at B) at C)` or `callsite(A at
callsite(B at C))`, the translation now is the same. The previous
solution did not provide this guarantee, which meant the callsite
construction would somehow impact this translation.

Author: zyx-billy

mlir/lib/Target/LLVMIR/DebugTranslation.cpp

@@ -406,10 +406,15 @@ llvm::DILocation *DebugTranslation::translateLoc(Location loc,
   if (auto callLoc = dyn_cast<CallSiteLoc>(loc)) {
     // For callsites, the caller is fed as the inlinedAt for the callee.
     auto *callerLoc = translateLoc(callLoc.getCaller(), scope, inlinedAt);
-    // If the caller scope does not exist, the callsite cannot be represented
-    // in LLVM (the callee scope may not match the function it is in).
-    if (!callerLoc)
-      return nullptr;
+    // If the caller scope is not translatable, the overall callsite cannot be
+    // represented in LLVM (the callee scope may not match the parent function).
+    if (!callerLoc) {
+      // If there is an inlinedAt scope (an outer caller), skip to that
+      // directly. Otherwise, cannot translate.
+      if (!inlinedAt)
+        return nullptr;
+      callerLoc = inlinedAt;
+    }
     llvmLoc = translateLoc(callLoc.getCallee(), nullptr, callerLoc);
     // Fallback: Ignore callee if it has no debug scope.
     if (!llvmLoc)

mlir/test/Target/LLVMIR/llvmir-debug.mlir

@@ -114,13 +114,13 @@ llvm.func @func_with_debug(%arg: i64) {
   // CHECK: call void @func_no_debug(), !dbg ![[MY_SOURCE_LOC]]
   llvm.call @func_no_debug() : () -> () loc(callsite("nodebug.cc":3:4 at fused<#sp0>["mysource.cc":5:6]))
 
-  // CHECK: call void @func_no_debug(), !dbg ![[MY_SOURCE_LOC]]
-  llvm.call @func_no_debug() : () -> () loc(callsite(callsite(fused<#callee>["nodebug.cc":3:4] at "foo.mlir":2:4) at fused<#sp0>["mysource.cc":5:6]))
-
   // CHECK: call void @func_no_debug(), !dbg ![[FUSED_LOC:[0-9]+]]
   llvm.call @func_no_debug() : () -> () loc(fused[callsite(fused<#callee>["mysource.cc":5:6] at "mysource.cc":1:1), "mysource.cc":1:1])
 
-  // CHECK: add i64 %[[ARG]], %[[ARG]], !dbg ![[FUSEDWITH_LOC:[0-9]+]]
+  // CHECK: call void @func_no_debug(), !dbg ![[FUSEDWITH_LOC:[0-9]+]]
+  llvm.call @func_no_debug() : () -> () loc(callsite(callsite(fused<#callee>["foo.mlir":2:4] at "foo.mlir":1:1) at fused<#sp0>["foo.mlir":28:5]))
+
+  // CHECK: add i64 %[[ARG]], %[[ARG]], !dbg ![[FUSEDWITH_LOC]]
   %sum = llvm.add %arg, %arg : i64 loc(callsite(fused<#callee>["foo.mlir":2:4] at fused<#sp0>["foo.mlir":28:5]))
 
   llvm.return