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

[Billy Laws via cfe-commits]

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@@ -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() ? "#" : "");
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bylaws wrote:

See #78916 

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