[IRGenDebugInfo] Allow multiple dbg.declare on an alloca

This is unnecessarily dropping debug info, as there is currently
no assertion in LLVM. The sharing of stack slot can happen
because of AllocStackHoisting, which is run at -Onone too.

(cherry picked from commit 2726d83)

Author: Snowy1803

lib/IRGen/IRGenDebugInfo.cpp

@@ -3401,11 +3401,6 @@ void IRGenDebugInfoImpl::emitDbgIntrinsic(
   auto DL =
       llvm::DILocation::get(IGM.getLLVMContext(), Line, Col, Scope, InlinedAt);
 
-  // An alloca may only be described by exactly one dbg.declare.
-  if (isa<llvm::AllocaInst>(Storage) &&
-      !llvm::FindDbgDeclareUses(Storage).empty())
-    return;
-
   // Fragment DIExpression cannot cover the whole variable
   // or going out-of-bound.
   if (auto Fragment = Expr->getFragmentInfo()) {

test/DebugInfo/inlined-generics-basic.swift

@@ -52,6 +52,7 @@ public class C<R> {
     // SIL: function_ref {{.*}}use{{.*}} scope [[F1G3H]]
     // IR: dbg.value(metadata ptr %[[ARG_S]], metadata ![[MD_1_0:[0-9]+]]
     // IR: %[[RS_PAIR:.*]] = alloca i8, i{{.*}} %
+    // IR: dbg.declare(metadata ptr %[[RS_PAIR]], metadata ![[GRS_U:[0-9]+]],
     // IR: dbg.declare(metadata ptr %[[RS_PAIR]], metadata ![[GRS_T:[0-9]+]],
     // IR: dbg.value(metadata ptr %[[ARG_0]], metadata ![[S:[0-9]+]]
     // IR: dbg.value(metadata ptr %[[ARG_0]], metadata ![[GS_T:[0-9]+]]
@@ -100,6 +101,8 @@ public class C<R> {
 // IR-DAG: ![[MD_1_0]] = !DILocalVariable(name: "$\CF\84_1_0"
 // IR-DAG: ![[GRS_T]] = !DILocalVariable(name: "t", {{.*}} scope: ![[SP_GRS_T:[0-9]+]], {{.*}}type: ![[LET_TUPLE:[0-9]+]]
 // IR-DAG: ![[SP_GRS_T]] = {{.*}}linkageName: "$s1A1gyyxlFx_qd__t_Ti5"
+// IR-DAG: ![[GRS_U]] = !DILocalVariable(name: "u", {{.*}} scope: ![[SP_GRS_U:[0-9]+]], {{.*}}type: ![[LET_TUPLE:[0-9]+]]
+// IR-DAG: ![[SP_GRS_U]] = {{.*}}linkageName: "$s1A1hyyxlFx_qd__t_Ti5"
 // IR-DAG: ![[LET_TUPLE]] = !DIDerivedType(tag: DW_TAG_const_type, baseType: ![[TUPLE:[0-9]+]])
 // IR-DAG: ![[TUPLE]] = {{.*}}DW_TAG_structure_type, name: "$sx_qd__tD"
 // IR-DAG: ![[S]] = !DILocalVariable(name: "s", {{.*}} type: ![[LET_TAU_1_0:[0-9]+]]