[clang] [Clang][AArch64] Define __USER_LABEL_PREFIX__ to # for ARM64EC (PR #78913)

[Jon Roelofs via cfe-commits]

================
@@ -1462,10 +1462,12 @@ WindowsARM64TargetInfo::WindowsARM64TargetInfo(const llvm::Triple &Triple,
 }
 
 void WindowsARM64TargetInfo::setDataLayout() {
-  resetDataLayout(Triple.isOSBinFormatMachO()
-                      ? "e-m:o-i64:64-i128:128-n32:64-S128"
-                      : "e-m:w-p:64:64-i32:32-i64:64-i128:128-n32:64-S128",
-                  Triple.isOSBinFormatMachO() ? "_" : "");
+  if (Triple.isOSBinFormatMachO()) {
+    resetDataLayout("e-m:o-i64:64-i128:128-n32:64-S128", "_");
+  } else {
+    resetDataLayout("e-m:w-p:64:64-i32:32-i64:64-i128:128-n32:64-S128",
+                    Triple.isWindowsArm64EC() ? "#" : "");
----------------
jroelofs wrote:

Ok, mind adding just this part to _this_ patch?

```
// RUN: %clang_cc1 -E -dM -ffreestanding -triple=arm64ec-windows-msvc < /dev/null | FileCheck -match-full-lines -check-prefix ARM64EC-MSVC %s

 // ARM64EC-MSVC: #define __USER_LABEL_PREFIX__ #
```

https://github.com/llvm/llvm-project/pull/78913