[TBAA] Test for tbaa.struct metadata with bitfields in big endian layout (#87617)

This test exposes what I think is invalid tbaa.struct metadata currently
generated for bitfields when using big endian layout. The regions given
by `!{i64 2, i64 4, [[META3:![0-9]+]], i64 4, i64 4 ...` are
overlapping. This issue was originally observed in
#86709.

Author: juliannagele

clang/test/CodeGen/tbaa-struct-bitfield-endianness.cpp

@@ -0,0 +1,39 @@
+// RUN: %clang_cc1 -triple aarch64_be-apple-darwin -emit-llvm -o - -O1 %s | \
+// RUN:     FileCheck -check-prefixes=CHECK,CHECK-BE %s
+// RUN: %clang_cc1 -triple aarch64-apple-darwin -emit-llvm -o - -O1 %s | \
+// RUN:     FileCheck -check-prefixes=CHECK,CHECK-LE %s
+//
+// Check that TBAA metadata for structs containing bitfields is
+// consistent between big and little endian layouts.
+//
+// FIXME: The metadata below is invalid for the big endian layout: the
+// start offset of 2 is incorrect.
+
+struct NamedBitfields {
+  int f1 : 8;
+  int f2 : 8;
+  unsigned f3 : 1;
+  unsigned f4 : 15;
+  int f5;
+  double f6;
+};
+
+// CHECK-LABEL: _Z4copyP14NamedBitfieldsS0_
+// CHECK-SAME: ptr nocapture noundef writeonly [[A1:%.*]], ptr nocapture noundef readonly [[A2:%.*]]) local_unnamed_addr #[[ATTR0:[0-9]+]] {
+// CHECK-NEXT:  entry:
+// CHECK-NEXT:    tail call void @llvm.memcpy.p0.p0.i64(ptr noundef nonnull align 8 dereferenceable(16) [[A1]], ptr noundef nonnull align 8 dereferenceable(16) [[A2]], i64 16, i1 false), !tbaa.struct [[TBAA_STRUCT2:![0-9]+]]
+// CHECK-NEXT:    ret void
+//
+void copy(NamedBitfields *a1, NamedBitfields *a2) {
+  *a1 = *a2;
+}
+
+// CHECK-BE: [[TBAA_STRUCT2]] = !{i64 2, i64 4, [[META3:![0-9]+]], i64 4, i64 4, [[META6:![0-9]+]], i64 8, i64 8, [[META8:![0-9]+]]}
+// CHECK-LE: [[TBAA_STRUCT2]] = !{i64 0, i64 4, [[META3:![0-9]+]], i64 4, i64 4, [[META6:![0-9]+]], i64 8, i64 8, [[META8:![0-9]+]]}
+// CHECK: [[META3]] = !{[[META4:![0-9]+]], [[META4]], i64 0}
+// CHECK: [[META4]] = !{!"omnipotent char", [[META5:![0-9]+]], i64 0}
+// CHECK: [[META5]] = !{!"Simple C++ TBAA"}
+// CHECK: [[META6]] = !{[[META7:![0-9]+]], [[META7]], i64 0}
+// CHECK: [[META7]] = !{!"int", [[META4]], i64 0}
+// CHECK: [[META8]] = !{[[META9:![0-9]+]], [[META9]], i64 0}
+// CHECK: [[META9]] = !{!"double", [[META4]], i64 0}