mac/arm: XFAIL the last 3 failing tests

We should fix them, but let's XFAIL them for now so that we can start
running check-clang on bots and lock in the passing tests.

Part of 46644.

clang/test/CodeGen/2004-02-13-IllegalVararg.c

@@ -1,5 +1,8 @@
 // RUN: %clang_cc1  %s -w -emit-llvm -o -
 
+// https://bugs.llvm.org/show_bug.cgi?id=46644#c6
+// XFAIL: arm64-apple
+
 float test(int X, ...) {
   __builtin_va_list ap;
   float F;

clang/test/Driver/aarch64-cpus.c

@@ -1,5 +1,8 @@
 // Check target CPUs are correctly passed.
 
+// https://PR46644
+// XFAIL: arm64-apple
+
 // RUN: %clang -target aarch64 -### -c %s 2>&1 | FileCheck -check-prefix=GENERIC %s
 // RUN: %clang -target aarch64 -mcpu=generic -### -c %s 2>&1 | FileCheck -check-prefix=GENERIC %s
 // RUN: %clang -target aarch64 -mlittle-endian -### -c %s 2>&1 | FileCheck -check-prefix=GENERIC %s

clang/test/Index/pch-from-libclang.c

@@ -1,5 +1,8 @@
 // Check that clang can use a PCH created from libclang.
 
+// https://PR46644
+// XFAIL: arm64-apple
+
 // This test doesn't use -fdisable-module-hash and hence requires that
 // CompilerInvocation::getModuleHash() computes exactly the same hash
 // for c-index-test and clang, which in turn requires that the both use