[LoongArch][test] Add some ABI regression tests for empty struct. NFC

How empty structs (not as fields of container struct) are passed in C++
is not explicitly documented in psABI. This patch adds some tests
showing the current handing of clang. Some of the results are different
from gcc. Following patch(es) will try to fix the mismatch.

Author: SixWeining

clang/test/CodeGen/LoongArch/abi-lp64d-empty-structs.c

@@ -81,9 +81,62 @@ struct s8 test_s8(struct s8 a) {
   return a;
 }
 
+/// Note: Below tests check how empty structs are passed while above tests check
+/// empty structs as fields of container struct are ignored when flattening
+/// structs to examine whether the container structs can be passed via FARs.
+
 // CHECK-C: define{{.*}} void @test_s9()
 // CHECK-CXX: define{{.*}} i64 @_Z7test_s92s9(i64 {{.*}})
 struct s9 { struct empty e; };
 struct s9 test_s9(struct s9 a) {
   return a;
 }
+
+// CHECK-C: define{{.*}} void @test_s10()
+// CHECK-CXX: define{{.*}} void @_Z8test_s103s10()
+struct s10 { };
+struct s10 test_s10(struct s10 a) {
+  return a;
+}
+
+// CHECK-C: define{{.*}} void @test_s11()
+// CHECK-CXX: define{{.*}} i64 @_Z8test_s113s11(i64 {{.*}})
+struct s11 { struct { } s; };
+struct s11 test_s11(struct s11 a) {
+  return a;
+}
+
+// CHECK-C: define{{.*}} void @test_s12()
+// CHECK-CXX: define{{.*}} void @_Z8test_s123s12()
+struct s12 { int i[0]; };
+struct s12 test_s12(struct s12 a) {
+  return a;
+}
+
+// CHECK-C: define{{.*}} void @test_s13()
+// CHECK-CXX: define{{.*}} void @_Z8test_s133s13()
+struct s13 { struct { } s[0]; };
+struct s13 test_s13(struct s13 a) {
+  return a;
+}
+
+// CHECK-C: define{{.*}} void @test_s14()
+// CHECK-CXX: define{{.*}} i64 @_Z8test_s143s14(i64 {{.*}})
+struct s14 { struct { } s[1]; };
+struct s14 test_s14(struct s14 a) {
+  return a;
+}
+
+// CHECK-C: define{{.*}} void @test_s15()
+// CHECK-CXX: define{{.*}} void @_Z8test_s153s15()
+struct s15 { int : 0; };
+struct s15 test_s15(struct s15 a) {
+  return a;
+}
+
+// CHECK-C: define{{.*}} void @test_s16()
+// CHECK-CXX: define{{.*}} void @_Z8test_s163s16()
+struct s16 { int : 1; };
+struct s16 test_s16(struct s16 a) {
+  return a;
+}