[DebugInfo] Fix verifier crash for complex switch

An alloc_stack, just like a debug_value, should ignore location
overrides for their variable.

Fixes #73338
rdar://127348128

include/swift/SIL/SILBuilder.h

@@ -423,6 +423,9 @@ public:
 #else
     (void)skipVarDeclAssert;
 #endif
+    // Don't apply location overrides on variables.
+    if (Var && !Var->Loc)
+      Var->Loc = Loc;
     return insert(AllocStackInst::create(
         getSILDebugLocation(Loc, true), elementType, getFunction(),
         substituteAnonymousArgs(Name, Var, Loc), dynamic, isLexical,

test/DebugInfo/case-match-vars.swift

@@ -0,0 +1,45 @@
+// RUN: %empty-directory(%t)
+// RUN: %{python} %utils/split_file.py -o %t %s
+
+// RUN: %target-swift-frontend -module-name a -parse-as-library -emit-ir -g %t/Seq.swift %t/A.swift | %FileCheck %s
+
+// This code used to trigger the verifier.
+// https://github.com/apple/swift/issues/73338
+
+// BEGIN Seq.swift
+struct A<Element: Equatable>: ExpressibleByArrayLiteral, Equatable {
+  var base: Element?
+  init(arrayLiteral elements: Element...) {}
+}
+struct B<Element: Equatable>: ExpressibleByArrayLiteral, Equatable {
+  var first: Element?
+  init(arrayLiteral elements: Element...) {}
+}
+
+// BEGIN A.swift
+enum E<T: P> {
+  case a(A<T.ID>)
+  case b(B<T.ID>)
+  case c
+
+  static func ==(lhs: Self, rhs: Self) -> Bool {
+    switch (lhs, rhs) {
+    case (.a([]), .c), (.c, .a([])),
+         (.b([]), .c), (.c, .b([])):
+      return true
+    default:
+      return false
+    }
+  }
+}
+public protocol P {
+  associatedtype ID: Equatable
+}
+
+// The [] expressions should be available in the debugger
+
+// CHECK: !DILocalVariable(name: "$_0", {{.+}} line: 9
+// CHECK: !DILocalVariable(name: "$_1", {{.+}} line: 9
+// CHECK: !DILocalVariable(name: "$_2", {{.+}} line: 8
+// CHECK: !DILocalVariable(name: "$_3", {{.+}} line: 8
+