[Clang][DebugInfo] Clang generates an extra spurious unnamed 'dbg.declare'.

Do not emit call to llvm.dbg.declare when the variable declaration
is a DecompositionDecl as its instance class is always unnamed.

The emitted debug declare looks like:
  call void @llvm.dbg.declare(metadata ..., metadata !xx, metadata ...)
  !xx = !DILocalVariable(scope: !..., file: !..., line: ..., type: !...)

- Added check for missing captured 'a' variable.
- Check for the specific values in DW_OP_plus_uconst.

Author: CarlosAlbertoEnciso

clang/test/CodeGenCXX/debug-info-structured-binding.cpp

@@ -1,10 +1,11 @@
 // RUN: %clang_cc1 -emit-llvm -debug-info-kind=standalone -triple %itanium_abi_triple %s -o - | FileCheck %s --implicit-check-not="call void @llvm.dbg.declare"
 
-// CHECK: call void @llvm.dbg.declare(metadata ptr %{{[a-z]+}}, metadata !{{[0-9]+}}, metadata !DIExpression())
+// CHECK: call void @llvm.dbg.declare(metadata ptr %{{[a-z]+}}, metadata ![[VAR_0:[0-9]+]], metadata !DIExpression())
 // CHECK: call void @llvm.dbg.declare(metadata ptr %{{[0-9]+}}, metadata ![[VAR_1:[0-9]+]], metadata !DIExpression())
-// CHECK: call void @llvm.dbg.declare(metadata ptr %{{[0-9]+}}, metadata ![[VAR_2:[0-9]+]], metadata !DIExpression(DW_OP_plus_uconst, {{[0-9]+}}))
+// CHECK: call void @llvm.dbg.declare(metadata ptr %{{[0-9]+}}, metadata ![[VAR_2:[0-9]+]], metadata !DIExpression(DW_OP_plus_uconst, 4))
 // CHECK: call void @llvm.dbg.declare(metadata ptr %{{[0-9]+}}, metadata ![[VAR_3:[0-9]+]], metadata !DIExpression(DW_OP_deref))
-// CHECK: call void @llvm.dbg.declare(metadata ptr %{{[0-9]+}}, metadata ![[VAR_4:[0-9]+]], metadata !DIExpression(DW_OP_deref, DW_OP_plus_uconst, {{[0-9]+}}))
+// CHECK: call void @llvm.dbg.declare(metadata ptr %{{[0-9]+}}, metadata ![[VAR_4:[0-9]+]], metadata !DIExpression(DW_OP_deref, DW_OP_plus_uconst, 4))
+// CHECK: ![[VAR_0]] = !DILocalVariable(name: "a"
 // CHECK: ![[VAR_1]] = !DILocalVariable(name: "x1"
 // CHECK: ![[VAR_2]] = !DILocalVariable(name: "y1"
 // CHECK: ![[VAR_3]] = !DILocalVariable(name: "x2"