[CIR][CodeGen] Fix array initialization in CIRGenExprAgg (#852)

Mistakenly closed #850

#850 (review)
 
This PR fixes array initialization for expression arguments. 

Consider the following code snippet `test.c`: 
```
typedef struct {
  int a;
  int b[2];
} A;

int bar() {
  return 42;
}

void foo() {
  A a = {bar(), {}};
}
```
When ran with `bin/clang test.c -Xclang -fclangir -Xclang -emit-cir -S
-o -`, It produces the following error:
```
~/clangir/clang/lib/CIR/CodeGen/CIRGenExprAgg.cpp:483: void {anonymous}::AggExprEmitter::buildArrayInit(cir::Address, mlir::cir::ArrayType, clang::QualType, clang::Expr*, llvm::ArrayRef<clang::Expr*>, clang::Expr*): Assertion `NumInitElements != 0' failed.
```
The error can be traced back to `CIRGenExprAgg.cpp`, and the fix is
simple. It is possible to have an empty array initialization as an
expression argument!

Author: bruteforceboy

clang/lib/CIR/CodeGen/CIRGenExprAgg.cpp

@@ -480,7 +480,6 @@ void AggExprEmitter::buildArrayInit(Address DestPtr, mlir::cir::ArrayType AType,
   uint64_t NumInitElements = Args.size();
 
   uint64_t NumArrayElements = AType.getSize();
-  assert(NumInitElements != 0 && "expected at least one initializaed value");
   assert(NumInitElements <= NumArrayElements);
 
   QualType elementType =

clang/test/CIR/CodeGen/array-init.cpp

@@ -0,0 +1,38 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -std=c++17 -fclangir -emit-cir %s -o %t.cir
+// RUN: FileCheck --input-file=%t.cir %s
+
+typedef struct {
+  int a;
+  int b[2];
+} A;
+
+int bar() {
+  return 42;
+}
+
+void foo() {
+  A a = {bar(), {}};
+}
+// CHECK: %[[VAL_0:.*]] = cir.alloca !ty_A, !cir.ptr<!ty_A>, ["a", init]
+// CHECK: %[[VAL_1:.*]] = cir.alloca !cir.ptr<!s32i>, !cir.ptr<!cir.ptr<!s32i>>, ["arrayinit.temp", init]
+// CHECK: %[[VAL_2:.*]] = cir.get_member %[[VAL_0]][0] {name = "a"} : !cir.ptr<!ty_A> -> !cir.ptr<!s32i>
+// CHECK: %[[VAL_3:.*]] = cir.call @_Z3barv() : () -> !s32i
+// CHECK: cir.store %[[VAL_3]], %[[VAL_2]] : !s32i, !cir.ptr<!s32i>
+// CHECK: %[[VAL_4:.*]] = cir.get_member %[[VAL_0]][1] {name = "b"} : !cir.ptr<!ty_A> -> !cir.ptr<!cir.array<!s32i x 2>>
+// CHECK: %[[VAL_5:.*]] = cir.cast(array_to_ptrdecay, %[[VAL_4]] : !cir.ptr<!cir.array<!s32i x 2>>), !cir.ptr<!s32i>
+// CHECK: cir.store %[[VAL_5]], %[[VAL_1]] : !cir.ptr<!s32i>, !cir.ptr<!cir.ptr<!s32i>>
+// CHECK: %[[VAL_6:.*]] = cir.const #cir.int<2> : !s64i
+// CHECK: %[[VAL_7:.*]] = cir.ptr_stride(%[[VAL_5]] : !cir.ptr<!s32i>, %[[VAL_6]] : !s64i), !cir.ptr<!s32i>
+// CHECK: cir.do {
+// CHECK:     %[[VAL_8:.*]] = cir.load %[[VAL_1]] : !cir.ptr<!cir.ptr<!s32i>>, !cir.ptr<!s32i>
+// CHECK:     %[[VAL_9:.*]] = cir.const #cir.int<0> : !s32i
+// CHECK:     cir.store %[[VAL_9]], %[[VAL_8]] : !s32i, !cir.ptr<!s32i>
+// CHECK:     %[[VAL_10:.*]] = cir.const #cir.int<1> : !s64i
+// CHECK:     %[[VAL_11:.*]] = cir.ptr_stride(%[[VAL_8]] : !cir.ptr<!s32i>, %[[VAL_10]] : !s64i), !cir.ptr<!s32i>
+// CHECK:     cir.store %[[VAL_11]], %[[VAL_1]] : !cir.ptr<!s32i>, !cir.ptr<!cir.ptr<!s32i>>
+// CHECK:     cir.yield
+// CHECK: } while {
+// CHECK:     %[[VAL_8:.*]] = cir.load %[[VAL_1]] : !cir.ptr<!cir.ptr<!s32i>>, !cir.ptr<!s32i>
+// CHECK:     %[[VAL_9:.*]] = cir.cmp(ne, %[[VAL_8]], %[[VAL_7]]) : !cir.ptr<!s32i>, !cir.bool
+// CHECK:     cir.condition(%[[VAL_9]])
+// CHECK: }