[SILOptimizer] Remove auxiliary debug scope when running SILDebugInfoGenerator

Previously inside SILDebugInfoGenerator (i.e. `-sil-based-debuginfo`
flag) we're removing all `debug_value` and `debug_value_addr` to avoid
mixing Swift and SIL source location. But we forgot to handle the source
location inside the auxiliary debug variable info in a `alloc_stack`
instruction. Which triggered a verification error regarding debug scope
mismatch.

This patch teaches SILDebugInfoGenerator to remove any auxiliary debug
variable scope, if there is any. Which might degrade the debug info
quality but we're mostly care about line number in this mode.

rdar://81709398

Author: mshockwave

lib/SILOptimizer/UtilityPasses/SILDebugInfoGenerator.cpp

@@ -150,6 +150,12 @@ class SILDebugInfoGenerator : public SILModuleTransform {
               I->eraseFromParent();
               continue;
             }
+            if (auto *ASI = dyn_cast<AllocStackInst>(I))
+              // Remove the debug variable scope enclosed
+              // within the SILDebugVariable such that we won't
+              // trigger a verification error.
+              ASI->setDebugVarScope(nullptr);
+
             SILLocation Loc = I->getLoc();
             auto *filePos = SILLocation::FilenameAndLocation::alloc(
                 Ctx.LineNums[I], 1, FileNameBuf, *M);

test/DebugInfo/sil_based_dbg.swift

@@ -15,3 +15,25 @@ public func testit() {
   print("Hello")
 }
 
+// We need to remove the debug scope within alloc_stack's auxiliary debug var info
+// in sil-based-dbg mode.
+// To create something like `alloc_stack ..., (name "foo", loc ..., scope 0)...`
+// as our testing input, we're only running SROA over the input swift code.
+// RUN: %target-swift-frontend %s -disable-debugger-shadow-copies -emit-sil -g -o %t/stage1.sil
+// RUN: %target-sil-opt -sil-print-debuginfo -access-marker-elim -sroa %t/stage1.sil -o %t/stage2.sil
+// The verification shouldn't fail
+// RUN: %target-swift-frontend %t/stage2.sil -sil-verify-all -sil-based-debuginfo -g -emit-sil -o %t/out.sil
+// RUN: %FileCheck %s --check-prefix=CHECK_DBG_SCOPE < %t/out.sil
+struct TheStruct {
+    var the_member : Int
+}
+// CHECK_DBG_SCOPE-LABEL: sil {{.*}}test_debug_scope
+public func test_debug_scope(val : Int) -> Int {
+    // CHECK_DBG_SCOPE: alloc_stack $Builtin.Int{{[0-9]+}}, var, (name "the_struct",
+    // CHECK_DBG_SCOPE-SAME:                                      loc
+    // The auxiliary debug scope should be removed
+    // CHECK_DBG_SCOPE-NOT:                                       scope {{[0-9]+}})
+    var the_struct = TheStruct(the_member: 0)
+    the_struct.the_member = val + 13
+    return the_struct.the_member
+}